src/HOL/Ring_and_Field.thy

linorder_neqE_ordered_idom: proper proof, avoid illegal schematic type vars;

(*  Title:   HOL/Ring_and_Field.thy
    ID:      $Id$
    Author:  Gertrud Bauer, Steven Obua, Tobias Nipkow, Lawrence C Paulson, and Markus Wenzel,
             with contributions by Jeremy Avigad
*)

header {* (Ordered) Rings and Fields *}

theory Ring_and_Field
imports OrderedGroup
begin

text {*
  The theory of partially ordered rings is taken from the books:
  \begin{itemize}
  \item \emph{Lattice Theory} by Garret Birkhoff, American Mathematical Society 1979 
  \item \emph{Partially Ordered Algebraic Systems}, Pergamon Press 1963
  \end{itemize}
  Most of the used notions can also be looked up in 
  \begin{itemize}
  \item \url{http://www.mathworld.com} by Eric Weisstein et. al.
  \item \emph{Algebra I} by van der Waerden, Springer.
  \end{itemize}
*}

class semiring = ab_semigroup_add + semigroup_mult +
  assumes left_distrib: "(a \<^loc>+ b) \<^loc>* c = a \<^loc>* c \<^loc>+ b \<^loc>* c"
  assumes right_distrib: "a \<^loc>* (b \<^loc>+ c) = a \<^loc>* b \<^loc>+ a \<^loc>* c"

class mult_zero = times + zero +
  assumes mult_zero_left [simp]: "\<^loc>0 \<^loc>* a = \<^loc>0"
  assumes mult_zero_right [simp]: "a \<^loc>* \<^loc>0 = \<^loc>0"

class semiring_0 = semiring + comm_monoid_add + mult_zero

class semiring_0_cancel = semiring + comm_monoid_add + cancel_ab_semigroup_add

instance semiring_0_cancel \<subseteq> semiring_0
proof
  fix a :: 'a
  have "0 * a + 0 * a = 0 * a + 0"
    by (simp add: left_distrib [symmetric])
  thus "0 * a = 0"
    by (simp only: add_left_cancel)

  have "a * 0 + a * 0 = a * 0 + 0"
    by (simp add: right_distrib [symmetric])
  thus "a * 0 = 0"
    by (simp only: add_left_cancel)
qed

class comm_semiring = ab_semigroup_add + ab_semigroup_mult +
  assumes distrib: "(a \<^loc>+ b) \<^loc>* c = a \<^loc>* c \<^loc>+ b \<^loc>* c"

instance comm_semiring \<subseteq> semiring
proof
  fix a b c :: 'a
  show "(a + b) * c = a * c + b * c" by (simp add: distrib)
  have "a * (b + c) = (b + c) * a" by (simp add: mult_ac)
  also have "... = b * a + c * a" by (simp only: distrib)
  also have "... = a * b + a * c" by (simp add: mult_ac)
  finally show "a * (b + c) = a * b + a * c" by blast
qed

class comm_semiring_0 = comm_semiring + comm_monoid_add + mult_zero

instance comm_semiring_0 \<subseteq> semiring_0 ..

class comm_semiring_0_cancel = comm_semiring + comm_monoid_add + cancel_ab_semigroup_add

instance comm_semiring_0_cancel \<subseteq> semiring_0_cancel ..

instance comm_semiring_0_cancel \<subseteq> comm_semiring_0 ..

class zero_neq_one = zero + one +
  assumes zero_neq_one [simp]: "\<^loc>0 \<noteq> \<^loc>1"

class semiring_1 = zero_neq_one + semiring_0 + monoid_mult

class comm_semiring_1 = zero_neq_one + comm_semiring_0 + comm_monoid_mult
  (*previously almost_semiring*)

instance comm_semiring_1 \<subseteq> semiring_1 ..

class no_zero_divisors = zero + times +
  assumes no_zero_divisors: "a \<noteq> \<^loc>0 \<Longrightarrow> b \<noteq> \<^loc>0 \<Longrightarrow> a \<^loc>* b \<noteq> \<^loc>0"

class semiring_1_cancel = semiring + comm_monoid_add + zero_neq_one
  + cancel_ab_semigroup_add + monoid_mult

instance semiring_1_cancel \<subseteq> semiring_0_cancel ..

instance semiring_1_cancel \<subseteq> semiring_1 ..

class comm_semiring_1_cancel = comm_semiring + comm_monoid_add + comm_monoid_mult
  + zero_neq_one + cancel_ab_semigroup_add

instance comm_semiring_1_cancel \<subseteq> semiring_1_cancel ..

instance comm_semiring_1_cancel \<subseteq> comm_semiring_0_cancel ..

instance comm_semiring_1_cancel \<subseteq> comm_semiring_1 ..

class ring = semiring + ab_group_add

instance ring \<subseteq> semiring_0_cancel ..

class comm_ring = comm_semiring + ab_group_add

instance comm_ring \<subseteq> ring ..

instance comm_ring \<subseteq> comm_semiring_0_cancel ..

class ring_1 = ring + zero_neq_one + monoid_mult

instance ring_1 \<subseteq> semiring_1_cancel ..

class comm_ring_1 = comm_ring + zero_neq_one + comm_monoid_mult
  (*previously ring*)

instance comm_ring_1 \<subseteq> ring_1 ..

instance comm_ring_1 \<subseteq> comm_semiring_1_cancel ..

class ring_no_zero_divisors = ring + no_zero_divisors

class ring_1_no_zero_divisors = ring_1 + ring_no_zero_divisors

class idom = comm_ring_1 + no_zero_divisors

instance idom \<subseteq> ring_1_no_zero_divisors ..

class division_ring = ring_1 + inverse +
  assumes left_inverse [simp]:  "a \<noteq> \<^loc>0 \<Longrightarrow> inverse a \<^loc>* a = \<^loc>1"
  assumes right_inverse [simp]: "a \<noteq> \<^loc>0 \<Longrightarrow> a \<^loc>* inverse a = \<^loc>1"

instance division_ring \<subseteq> ring_1_no_zero_divisors
proof
  fix a b :: 'a
  assume a: "a \<noteq> 0" and b: "b \<noteq> 0"
  show "a * b \<noteq> 0"
  proof
    assume ab: "a * b = 0"
    hence "0 = inverse a * (a * b) * inverse b"
      by simp
    also have "\<dots> = (inverse a * a) * (b * inverse b)"
      by (simp only: mult_assoc)
    also have "\<dots> = 1"
      using a b by simp
    finally show False
      by simp
  qed
qed

class field = comm_ring_1 + inverse +
  assumes field_inverse:  "a \<noteq> 0 \<Longrightarrow> inverse a \<^loc>* a = \<^loc>1"
  assumes divide_inverse: "a \<^loc>/ b = a \<^loc>* inverse b"

instance field \<subseteq> division_ring
proof
  fix a :: 'a
  assume "a \<noteq> 0"
  thus "inverse a * a = 1" by (rule field_inverse)
  thus "a * inverse a = 1" by (simp only: mult_commute)
qed

instance field \<subseteq> idom ..

class division_by_zero = zero + inverse +
  assumes inverse_zero [simp]: "inverse \<^loc>0 = \<^loc>0"


subsection {* Distribution rules *}

text{*For the @{text combine_numerals} simproc*}
lemma combine_common_factor:
     "a*e + (b*e + c) = (a+b)*e + (c::'a::semiring)"
by (simp add: left_distrib add_ac)

lemma minus_mult_left: "- (a * b) = (-a) * (b::'a::ring)"
apply (rule equals_zero_I)
apply (simp add: left_distrib [symmetric]) 
done

lemma minus_mult_right: "- (a * b) = a * -(b::'a::ring)"
apply (rule equals_zero_I)
apply (simp add: right_distrib [symmetric]) 
done

lemma minus_mult_minus [simp]: "(- a) * (- b) = a * (b::'a::ring)"
  by (simp add: minus_mult_left [symmetric] minus_mult_right [symmetric])

lemma minus_mult_commute: "(- a) * b = a * (- b::'a::ring)"
  by (simp add: minus_mult_left [symmetric] minus_mult_right [symmetric])

lemma right_diff_distrib: "a * (b - c) = a * b - a * (c::'a::ring)"
by (simp add: right_distrib diff_minus 
              minus_mult_left [symmetric] minus_mult_right [symmetric]) 

lemma left_diff_distrib: "(a - b) * c = a * c - b * (c::'a::ring)"
by (simp add: left_distrib diff_minus 
              minus_mult_left [symmetric] minus_mult_right [symmetric]) 

lemmas ring_distribs =
  right_distrib left_distrib left_diff_distrib right_diff_distrib

text{*This list of rewrites simplifies ring terms by multiplying
everything out and bringing sums and products into a canonical form
(by ordered rewriting). As a result it decides ring equalities but
also helps with inequalities. *}
lemmas ring_simps = group_simps ring_distribs

class mult_mono = times + zero + ord +
  assumes mult_left_mono: "a \<sqsubseteq> b \<Longrightarrow> \<^loc>0 \<sqsubseteq> c \<Longrightarrow> c \<^loc>* a \<sqsubseteq> c \<^loc>* b"
  assumes mult_right_mono: "a \<sqsubseteq> b \<Longrightarrow> \<^loc>0 \<sqsubseteq> c \<Longrightarrow> a \<^loc>* c \<sqsubseteq> b \<^loc>* c"

class pordered_semiring = mult_mono + semiring_0 + pordered_ab_semigroup_add 

class pordered_cancel_semiring = mult_mono + pordered_ab_semigroup_add
  + semiring + comm_monoid_add + cancel_ab_semigroup_add

instance pordered_cancel_semiring \<subseteq> semiring_0_cancel ..

instance pordered_cancel_semiring \<subseteq> pordered_semiring .. 

class ordered_semiring = semiring + comm_monoid_add + ordered_cancel_ab_semigroup_add + mult_mono

instance ordered_semiring \<subseteq> pordered_cancel_semiring ..

class ordered_semiring_strict = semiring + comm_monoid_add + ordered_cancel_ab_semigroup_add +
  assumes mult_strict_left_mono: "a \<sqsubset> b \<Longrightarrow> \<^loc>0 \<sqsubset> c \<Longrightarrow> c \<^loc>* a \<sqsubset> c \<^loc>* b"
  assumes mult_strict_right_mono: "a \<sqsubset> b \<Longrightarrow> \<^loc>0 \<sqsubset> c \<Longrightarrow> a \<^loc>* c \<sqsubset> b \<^loc>* c"

instance ordered_semiring_strict \<subseteq> semiring_0_cancel ..

instance ordered_semiring_strict \<subseteq> ordered_semiring
proof
  fix a b c :: 'a
  assume A: "a \<le> b" "0 \<le> c"
  from A show "c * a \<le> c * b"
    unfolding order_le_less
    using mult_strict_left_mono by auto
  from A show "a * c \<le> b * c"
    unfolding order_le_less
    using mult_strict_right_mono by auto
qed

class mult_mono1 = times + zero + ord +
  assumes mult_mono: "a \<sqsubseteq> b \<Longrightarrow> \<^loc>0 \<sqsubseteq> c \<Longrightarrow> c \<^loc>* a \<sqsubseteq> c \<^loc>* b"

class pordered_comm_semiring = comm_semiring_0
  + pordered_ab_semigroup_add + mult_mono1

class pordered_cancel_comm_semiring = comm_semiring_0_cancel
  + pordered_ab_semigroup_add + mult_mono1
  
instance pordered_cancel_comm_semiring \<subseteq> pordered_comm_semiring ..

class ordered_comm_semiring_strict = comm_semiring_0 + ordered_cancel_ab_semigroup_add +
  assumes mult_strict_mono: "a \<sqsubset> b \<Longrightarrow> \<^loc>0 \<sqsubset> c \<Longrightarrow> c \<^loc>* a \<sqsubset> c \<^loc>* b"

instance pordered_comm_semiring \<subseteq> pordered_semiring
proof
  fix a b c :: 'a
  assume "a \<le> b" "0 \<le> c"
  thus "c * a \<le> c * b" by (rule mult_mono)
  thus "a * c \<le> b * c" by (simp only: mult_commute)
qed

instance pordered_cancel_comm_semiring \<subseteq> pordered_cancel_semiring ..

instance ordered_comm_semiring_strict \<subseteq> ordered_semiring_strict
proof
  fix a b c :: 'a
  assume "a < b" "0 < c"
  thus "c * a < c * b" by (rule mult_strict_mono)
  thus "a * c < b * c" by (simp only: mult_commute)
qed

instance ordered_comm_semiring_strict \<subseteq> pordered_cancel_comm_semiring
proof
  fix a b c :: 'a
  assume "a \<le> b" "0 \<le> c"
  thus "c * a \<le> c * b"
    unfolding order_le_less
    using mult_strict_mono by auto
qed

class pordered_ring = ring + pordered_cancel_semiring 

instance pordered_ring \<subseteq> pordered_ab_group_add ..

class lordered_ring = pordered_ring + lordered_ab_group_abs

instance lordered_ring \<subseteq> lordered_ab_group_meet ..

instance lordered_ring \<subseteq> lordered_ab_group_join ..

class abs_if = minus + ord + zero + abs +
  assumes abs_if: "abs a = (if a \<sqsubset> 0 then (uminus a) else a)"

class sgn_if = sgn + zero + one + minus + ord +
assumes sgn_if: "sgn x = (if x = 0 then 0 else if 0 \<sqsubset> x then 1 else uminus 1)"

(* The "strict" suffix can be seen as describing the combination of ordered_ring and no_zero_divisors.
   Basically, ordered_ring + no_zero_divisors = ordered_ring_strict.
 *)
class ordered_ring = ring + ordered_semiring + lordered_ab_group + abs_if

instance ordered_ring \<subseteq> lordered_ring
proof
  fix x :: 'a
  show "\<bar>x\<bar> = sup x (- x)"
    by (simp only: abs_if sup_eq_if)
qed

class ordered_ring_strict =
  ring + ordered_semiring_strict + lordered_ab_group + abs_if

instance ordered_ring_strict \<subseteq> ordered_ring ..

class pordered_comm_ring = comm_ring + pordered_comm_semiring

instance pordered_comm_ring \<subseteq> pordered_ring ..

instance pordered_comm_ring \<subseteq> pordered_cancel_comm_semiring ..

class ordered_semidom = comm_semiring_1_cancel + ordered_comm_semiring_strict +
  (*previously ordered_semiring*)
  assumes zero_less_one [simp]: "\<^loc>0 \<sqsubset> \<^loc>1"

lemma pos_add_strict:
  fixes a b c :: "'a\<Colon>ordered_semidom"
  shows "0 < a \<Longrightarrow> b < c \<Longrightarrow> b < a + c"
  using add_strict_mono [of 0 a b c] by simp

class ordered_idom =
  comm_ring_1 +
  ordered_comm_semiring_strict +
  lordered_ab_group +
  abs_if + sgn_if
  (*previously ordered_ring*)

instance ordered_idom \<subseteq> ordered_ring_strict ..

instance ordered_idom \<subseteq> pordered_comm_ring ..

class ordered_field = field + ordered_idom

lemma linorder_neqE_ordered_idom:
  fixes x y :: "'a :: ordered_idom"
  assumes "x \<noteq> y" obtains "x < y" | "y < x"
  using assms by (rule linorder_neqE)

lemma eq_add_iff1:
  "(a*e + c = b*e + d) = ((a-b)*e + c = (d::'a::ring))"
by (simp add: ring_simps)

lemma eq_add_iff2:
  "(a*e + c = b*e + d) = (c = (b-a)*e + (d::'a::ring))"
by (simp add: ring_simps)

lemma less_add_iff1:
  "(a*e + c < b*e + d) = ((a-b)*e + c < (d::'a::pordered_ring))"
by (simp add: ring_simps)

lemma less_add_iff2:
  "(a*e + c < b*e + d) = (c < (b-a)*e + (d::'a::pordered_ring))"
by (simp add: ring_simps)

lemma le_add_iff1:
  "(a*e + c \<le> b*e + d) = ((a-b)*e + c \<le> (d::'a::pordered_ring))"
by (simp add: ring_simps)

lemma le_add_iff2:
  "(a*e + c \<le> b*e + d) = (c \<le> (b-a)*e + (d::'a::pordered_ring))"
by (simp add: ring_simps)


subsection {* Ordering Rules for Multiplication *}

lemma mult_left_le_imp_le:
  "[|c*a \<le> c*b; 0 < c|] ==> a \<le> (b::'a::ordered_semiring_strict)"
by (force simp add: mult_strict_left_mono linorder_not_less [symmetric])
 
lemma mult_right_le_imp_le:
  "[|a*c \<le> b*c; 0 < c|] ==> a \<le> (b::'a::ordered_semiring_strict)"
by (force simp add: mult_strict_right_mono linorder_not_less [symmetric])

lemma mult_left_less_imp_less:
  "[|c*a < c*b; 0 \<le> c|] ==> a < (b::'a::ordered_semiring)"
by (force simp add: mult_left_mono linorder_not_le [symmetric])
 
lemma mult_right_less_imp_less:
  "[|a*c < b*c; 0 \<le> c|] ==> a < (b::'a::ordered_semiring)"
by (force simp add: mult_right_mono linorder_not_le [symmetric])

lemma mult_strict_left_mono_neg:
  "[|b < a; c < 0|] ==> c * a < c * (b::'a::ordered_ring_strict)"
apply (drule mult_strict_left_mono [of _ _ "-c"])
apply (simp_all add: minus_mult_left [symmetric]) 
done

lemma mult_left_mono_neg:
  "[|b \<le> a; c \<le> 0|] ==> c * a \<le>  c * (b::'a::pordered_ring)"
apply (drule mult_left_mono [of _ _ "-c"])
apply (simp_all add: minus_mult_left [symmetric]) 
done

lemma mult_strict_right_mono_neg:
  "[|b < a; c < 0|] ==> a * c < b * (c::'a::ordered_ring_strict)"
apply (drule mult_strict_right_mono [of _ _ "-c"])
apply (simp_all add: minus_mult_right [symmetric]) 
done

lemma mult_right_mono_neg:
  "[|b \<le> a; c \<le> 0|] ==> a * c \<le>  (b::'a::pordered_ring) * c"
apply (drule mult_right_mono [of _ _ "-c"])
apply (simp)
apply (simp_all add: minus_mult_right [symmetric]) 
done


subsection{* Products of Signs *}

lemma mult_pos_pos: "[| (0::'a::ordered_semiring_strict) < a; 0 < b |] ==> 0 < a*b"
by (drule mult_strict_left_mono [of 0 b], auto)

lemma mult_nonneg_nonneg: "[| (0::'a::pordered_cancel_semiring) \<le> a; 0 \<le> b |] ==> 0 \<le> a*b"
by (drule mult_left_mono [of 0 b], auto)

lemma mult_pos_neg: "[| (0::'a::ordered_semiring_strict) < a; b < 0 |] ==> a*b < 0"
by (drule mult_strict_left_mono [of b 0], auto)

lemma mult_nonneg_nonpos: "[| (0::'a::pordered_cancel_semiring) \<le> a; b \<le> 0 |] ==> a*b \<le> 0"
by (drule mult_left_mono [of b 0], auto)

lemma mult_pos_neg2: "[| (0::'a::ordered_semiring_strict) < a; b < 0 |] ==> b*a < 0" 
by (drule mult_strict_right_mono[of b 0], auto)

lemma mult_nonneg_nonpos2: "[| (0::'a::pordered_cancel_semiring) \<le> a; b \<le> 0 |] ==> b*a \<le> 0" 
by (drule mult_right_mono[of b 0], auto)

lemma mult_neg_neg: "[| a < (0::'a::ordered_ring_strict); b < 0 |] ==> 0 < a*b"
by (drule mult_strict_right_mono_neg, auto)

lemma mult_nonpos_nonpos: "[| a \<le> (0::'a::pordered_ring); b \<le> 0 |] ==> 0 \<le> a*b"
by (drule mult_right_mono_neg[of a 0 b ], auto)

lemma zero_less_mult_pos:
     "[| 0 < a*b; 0 < a|] ==> 0 < (b::'a::ordered_semiring_strict)"
apply (cases "b\<le>0") 
 apply (auto simp add: order_le_less linorder_not_less)
apply (drule_tac mult_pos_neg [of a b]) 
 apply (auto dest: order_less_not_sym)
done

lemma zero_less_mult_pos2:
     "[| 0 < b*a; 0 < a|] ==> 0 < (b::'a::ordered_semiring_strict)"
apply (cases "b\<le>0") 
 apply (auto simp add: order_le_less linorder_not_less)
apply (drule_tac mult_pos_neg2 [of a b]) 
 apply (auto dest: order_less_not_sym)
done

lemma zero_less_mult_iff:
     "((0::'a::ordered_ring_strict) < a*b) = (0 < a & 0 < b | a < 0 & b < 0)"
apply (auto simp add: order_le_less linorder_not_less mult_pos_pos 
  mult_neg_neg)
apply (blast dest: zero_less_mult_pos) 
apply (blast dest: zero_less_mult_pos2)
done

lemma mult_eq_0_iff [simp]:
  fixes a b :: "'a::ring_no_zero_divisors"
  shows "(a * b = 0) = (a = 0 \<or> b = 0)"
by (cases "a = 0 \<or> b = 0", auto dest: no_zero_divisors)

instance ordered_ring_strict \<subseteq> ring_no_zero_divisors
apply intro_classes
apply (auto simp add: linorder_not_less order_le_less linorder_neq_iff)
apply (force dest: mult_strict_right_mono_neg mult_strict_right_mono)+
done

lemma zero_le_mult_iff:
     "((0::'a::ordered_ring_strict) \<le> a*b) = (0 \<le> a & 0 \<le> b | a \<le> 0 & b \<le> 0)"
by (auto simp add: eq_commute [of 0] order_le_less linorder_not_less
                   zero_less_mult_iff)

lemma mult_less_0_iff:
     "(a*b < (0::'a::ordered_ring_strict)) = (0 < a & b < 0 | a < 0 & 0 < b)"
apply (insert zero_less_mult_iff [of "-a" b]) 
apply (force simp add: minus_mult_left[symmetric]) 
done

lemma mult_le_0_iff:
     "(a*b \<le> (0::'a::ordered_ring_strict)) = (0 \<le> a & b \<le> 0 | a \<le> 0 & 0 \<le> b)"
apply (insert zero_le_mult_iff [of "-a" b]) 
apply (force simp add: minus_mult_left[symmetric]) 
done

lemma split_mult_pos_le: "(0 \<le> a & 0 \<le> b) | (a \<le> 0 & b \<le> 0) \<Longrightarrow> 0 \<le> a * (b::_::pordered_ring)"
by (auto simp add: mult_nonneg_nonneg mult_nonpos_nonpos)

lemma split_mult_neg_le: "(0 \<le> a & b \<le> 0) | (a \<le> 0 & 0 \<le> b) \<Longrightarrow> a * b \<le> (0::_::pordered_cancel_semiring)" 
by (auto simp add: mult_nonneg_nonpos mult_nonneg_nonpos2)

lemma zero_le_square[simp]: "(0::'a::ordered_ring_strict) \<le> a*a"
by (simp add: zero_le_mult_iff linorder_linear)

lemma not_square_less_zero[simp]: "\<not> (a * a < (0::'a::ordered_ring_strict))"
by (simp add: not_less)

text{*Proving axiom @{text zero_less_one} makes all @{text ordered_semidom}
      theorems available to members of @{term ordered_idom} *}

instance ordered_idom \<subseteq> ordered_semidom
proof
  have "(0::'a) \<le> 1*1" by (rule zero_le_square)
  thus "(0::'a) < 1" by (simp add: order_le_less) 
qed

instance ordered_idom \<subseteq> idom ..

text{*All three types of comparision involving 0 and 1 are covered.*}

lemmas one_neq_zero = zero_neq_one [THEN not_sym]
declare one_neq_zero [simp]

lemma zero_le_one [simp]: "(0::'a::ordered_semidom) \<le> 1"
  by (rule zero_less_one [THEN order_less_imp_le]) 

lemma not_one_le_zero [simp]: "~ (1::'a::ordered_semidom) \<le> 0"
by (simp add: linorder_not_le) 

lemma not_one_less_zero [simp]: "~ (1::'a::ordered_semidom) < 0"
by (simp add: linorder_not_less) 


subsection{*More Monotonicity*}

text{*Strict monotonicity in both arguments*}
lemma mult_strict_mono:
     "[|a<b; c<d; 0<b; 0\<le>c|] ==> a * c < b * (d::'a::ordered_semiring_strict)"
apply (cases "c=0")
 apply (simp add: mult_pos_pos) 
apply (erule mult_strict_right_mono [THEN order_less_trans])
 apply (force simp add: order_le_less) 
apply (erule mult_strict_left_mono, assumption)
done

text{*This weaker variant has more natural premises*}
lemma mult_strict_mono':
     "[| a<b; c<d; 0 \<le> a; 0 \<le> c|] ==> a * c < b * (d::'a::ordered_semiring_strict)"
apply (rule mult_strict_mono)
apply (blast intro: order_le_less_trans)+
done

lemma mult_mono:
     "[|a \<le> b; c \<le> d; 0 \<le> b; 0 \<le> c|] 
      ==> a * c  \<le>  b * (d::'a::pordered_semiring)"
apply (erule mult_right_mono [THEN order_trans], assumption)
apply (erule mult_left_mono, assumption)
done

lemma mult_mono':
     "[|a \<le> b; c \<le> d; 0 \<le> a; 0 \<le> c|] 
      ==> a * c  \<le>  b * (d::'a::pordered_semiring)"
apply (rule mult_mono)
apply (fast intro: order_trans)+
done

lemma less_1_mult: "[| 1 < m; 1 < n |] ==> 1 < m*(n::'a::ordered_semidom)"
apply (insert mult_strict_mono [of 1 m 1 n]) 
apply (simp add:  order_less_trans [OF zero_less_one]) 
done

lemma mult_less_le_imp_less: "(a::'a::ordered_semiring_strict) < b ==>
    c <= d ==> 0 <= a ==> 0 < c ==> a * c < b * d"
  apply (subgoal_tac "a * c < b * c")
  apply (erule order_less_le_trans)
  apply (erule mult_left_mono)
  apply simp
  apply (erule mult_strict_right_mono)
  apply assumption
done

lemma mult_le_less_imp_less: "(a::'a::ordered_semiring_strict) <= b ==>
    c < d ==> 0 < a ==> 0 <= c ==> a * c < b * d"
  apply (subgoal_tac "a * c <= b * c")
  apply (erule order_le_less_trans)
  apply (erule mult_strict_left_mono)
  apply simp
  apply (erule mult_right_mono)
  apply simp
done


subsection{*Cancellation Laws for Relationships With a Common Factor*}

text{*Cancellation laws for @{term "c*a < c*b"} and @{term "a*c < b*c"},
   also with the relations @{text "\<le>"} and equality.*}

text{*These ``disjunction'' versions produce two cases when the comparison is
 an assumption, but effectively four when the comparison is a goal.*}

lemma mult_less_cancel_right_disj:
    "(a*c < b*c) = ((0 < c & a < b) | (c < 0 & b < (a::'a::ordered_ring_strict)))"
apply (cases "c = 0")
apply (auto simp add: linorder_neq_iff mult_strict_right_mono 
                      mult_strict_right_mono_neg)
apply (auto simp add: linorder_not_less 
                      linorder_not_le [symmetric, of "a*c"]
                      linorder_not_le [symmetric, of a])
apply (erule_tac [!] notE)
apply (auto simp add: order_less_imp_le mult_right_mono 
                      mult_right_mono_neg)
done

lemma mult_less_cancel_left_disj:
    "(c*a < c*b) = ((0 < c & a < b) | (c < 0 & b < (a::'a::ordered_ring_strict)))"
apply (cases "c = 0")
apply (auto simp add: linorder_neq_iff mult_strict_left_mono 
                      mult_strict_left_mono_neg)
apply (auto simp add: linorder_not_less 
                      linorder_not_le [symmetric, of "c*a"]
                      linorder_not_le [symmetric, of a])
apply (erule_tac [!] notE)
apply (auto simp add: order_less_imp_le mult_left_mono 
                      mult_left_mono_neg)
done


text{*The ``conjunction of implication'' lemmas produce two cases when the
comparison is a goal, but give four when the comparison is an assumption.*}

lemma mult_less_cancel_right:
  fixes c :: "'a :: ordered_ring_strict"
  shows      "(a*c < b*c) = ((0 \<le> c --> a < b) & (c \<le> 0 --> b < a))"
by (insert mult_less_cancel_right_disj [of a c b], auto)

lemma mult_less_cancel_left:
  fixes c :: "'a :: ordered_ring_strict"
  shows      "(c*a < c*b) = ((0 \<le> c --> a < b) & (c \<le> 0 --> b < a))"
by (insert mult_less_cancel_left_disj [of c a b], auto)

lemma mult_le_cancel_right:
     "(a*c \<le> b*c) = ((0<c --> a\<le>b) & (c<0 --> b \<le> (a::'a::ordered_ring_strict)))"
by (simp add: linorder_not_less [symmetric] mult_less_cancel_right_disj)

lemma mult_le_cancel_left:
     "(c*a \<le> c*b) = ((0<c --> a\<le>b) & (c<0 --> b \<le> (a::'a::ordered_ring_strict)))"
by (simp add: linorder_not_less [symmetric] mult_less_cancel_left_disj)

lemma mult_less_imp_less_left:
      assumes less: "c*a < c*b" and nonneg: "0 \<le> c"
      shows "a < (b::'a::ordered_semiring_strict)"
proof (rule ccontr)
  assume "~ a < b"
  hence "b \<le> a" by (simp add: linorder_not_less)
  hence "c*b \<le> c*a" using nonneg by (rule mult_left_mono)
  with this and less show False 
    by (simp add: linorder_not_less [symmetric])
qed

lemma mult_less_imp_less_right:
  assumes less: "a*c < b*c" and nonneg: "0 <= c"
  shows "a < (b::'a::ordered_semiring_strict)"
proof (rule ccontr)
  assume "~ a < b"
  hence "b \<le> a" by (simp add: linorder_not_less)
  hence "b*c \<le> a*c" using nonneg by (rule mult_right_mono)
  with this and less show False 
    by (simp add: linorder_not_less [symmetric])
qed  

text{*Cancellation of equalities with a common factor*}
lemma mult_cancel_right [simp,noatp]:
  fixes a b c :: "'a::ring_no_zero_divisors"
  shows "(a * c = b * c) = (c = 0 \<or> a = b)"
proof -
  have "(a * c = b * c) = ((a - b) * c = 0)"
    by (simp add: ring_distribs)
  thus ?thesis
    by (simp add: disj_commute)
qed

lemma mult_cancel_left [simp,noatp]:
  fixes a b c :: "'a::ring_no_zero_divisors"
  shows "(c * a = c * b) = (c = 0 \<or> a = b)"
proof -
  have "(c * a = c * b) = (c * (a - b) = 0)"
    by (simp add: ring_distribs)
  thus ?thesis
    by simp
qed


subsubsection{*Special Cancellation Simprules for Multiplication*}

text{*These also produce two cases when the comparison is a goal.*}

lemma mult_le_cancel_right1:
  fixes c :: "'a :: ordered_idom"
  shows "(c \<le> b*c) = ((0<c --> 1\<le>b) & (c<0 --> b \<le> 1))"
by (insert mult_le_cancel_right [of 1 c b], simp)

lemma mult_le_cancel_right2:
  fixes c :: "'a :: ordered_idom"
  shows "(a*c \<le> c) = ((0<c --> a\<le>1) & (c<0 --> 1 \<le> a))"
by (insert mult_le_cancel_right [of a c 1], simp)

lemma mult_le_cancel_left1:
  fixes c :: "'a :: ordered_idom"
  shows "(c \<le> c*b) = ((0<c --> 1\<le>b) & (c<0 --> b \<le> 1))"
by (insert mult_le_cancel_left [of c 1 b], simp)

lemma mult_le_cancel_left2:
  fixes c :: "'a :: ordered_idom"
  shows "(c*a \<le> c) = ((0<c --> a\<le>1) & (c<0 --> 1 \<le> a))"
by (insert mult_le_cancel_left [of c a 1], simp)

lemma mult_less_cancel_right1:
  fixes c :: "'a :: ordered_idom"
  shows "(c < b*c) = ((0 \<le> c --> 1<b) & (c \<le> 0 --> b < 1))"
by (insert mult_less_cancel_right [of 1 c b], simp)

lemma mult_less_cancel_right2:
  fixes c :: "'a :: ordered_idom"
  shows "(a*c < c) = ((0 \<le> c --> a<1) & (c \<le> 0 --> 1 < a))"
by (insert mult_less_cancel_right [of a c 1], simp)

lemma mult_less_cancel_left1:
  fixes c :: "'a :: ordered_idom"
  shows "(c < c*b) = ((0 \<le> c --> 1<b) & (c \<le> 0 --> b < 1))"
by (insert mult_less_cancel_left [of c 1 b], simp)

lemma mult_less_cancel_left2:
  fixes c :: "'a :: ordered_idom"
  shows "(c*a < c) = ((0 \<le> c --> a<1) & (c \<le> 0 --> 1 < a))"
by (insert mult_less_cancel_left [of c a 1], simp)

lemma mult_cancel_right1 [simp]:
  fixes c :: "'a :: ring_1_no_zero_divisors"
  shows "(c = b*c) = (c = 0 | b=1)"
by (insert mult_cancel_right [of 1 c b], force)

lemma mult_cancel_right2 [simp]:
  fixes c :: "'a :: ring_1_no_zero_divisors"
  shows "(a*c = c) = (c = 0 | a=1)"
by (insert mult_cancel_right [of a c 1], simp)
 
lemma mult_cancel_left1 [simp]:
  fixes c :: "'a :: ring_1_no_zero_divisors"
  shows "(c = c*b) = (c = 0 | b=1)"
by (insert mult_cancel_left [of c 1 b], force)

lemma mult_cancel_left2 [simp]:
  fixes c :: "'a :: ring_1_no_zero_divisors"
  shows "(c*a = c) = (c = 0 | a=1)"
by (insert mult_cancel_left [of c a 1], simp)


text{*Simprules for comparisons where common factors can be cancelled.*}
lemmas mult_compare_simps =
    mult_le_cancel_right mult_le_cancel_left
    mult_le_cancel_right1 mult_le_cancel_right2
    mult_le_cancel_left1 mult_le_cancel_left2
    mult_less_cancel_right mult_less_cancel_left
    mult_less_cancel_right1 mult_less_cancel_right2
    mult_less_cancel_left1 mult_less_cancel_left2
    mult_cancel_right mult_cancel_left
    mult_cancel_right1 mult_cancel_right2
    mult_cancel_left1 mult_cancel_left2


subsection {* Fields *}

lemma right_inverse_eq: "b \<noteq> 0 ==> (a / b = 1) = (a = (b::'a::field))"
proof
  assume neq: "b \<noteq> 0"
  {
    hence "a = (a / b) * b" by (simp add: divide_inverse mult_ac)
    also assume "a / b = 1"
    finally show "a = b" by simp
  next
    assume "a = b"
    with neq show "a / b = 1" by (simp add: divide_inverse)
  }
qed

lemma nonzero_inverse_eq_divide: "a \<noteq> 0 ==> inverse (a::'a::field) = 1/a"
by (simp add: divide_inverse)

lemma divide_self[simp]: "a \<noteq> 0 ==> a / (a::'a::field) = 1"
  by (simp add: divide_inverse)

lemma divide_zero [simp]: "a / 0 = (0::'a::{field,division_by_zero})"
by (simp add: divide_inverse)

lemma divide_self_if [simp]:
     "a / (a::'a::{field,division_by_zero}) = (if a=0 then 0 else 1)"
  by (simp add: divide_self)

lemma divide_zero_left [simp]: "0/a = (0::'a::field)"
by (simp add: divide_inverse)

lemma inverse_eq_divide: "inverse (a::'a::field) = 1/a"
by (simp add: divide_inverse)

lemma add_divide_distrib: "(a+b)/(c::'a::field) = a/c + b/c"
by (simp add: divide_inverse ring_distribs) 

(* what ordering?? this is a straight instance of mult_eq_0_iff
text{*Compared with @{text mult_eq_0_iff}, this version removes the requirement
      of an ordering.*}
lemma field_mult_eq_0_iff [simp]:
  "(a*b = (0::'a::division_ring)) = (a = 0 | b = 0)"
by simp
*)
(* subsumed by mult_cancel lemmas on ring_no_zero_divisors
text{*Cancellation of equalities with a common factor*}
lemma field_mult_cancel_right_lemma:
      assumes cnz: "c \<noteq> (0::'a::division_ring)"
         and eq:  "a*c = b*c"
        shows "a=b"
proof -
  have "(a * c) * inverse c = (b * c) * inverse c"
    by (simp add: eq)
  thus "a=b"
    by (simp add: mult_assoc cnz)
qed

lemma field_mult_cancel_right [simp]:
     "(a*c = b*c) = (c = (0::'a::division_ring) | a=b)"
by simp

lemma field_mult_cancel_left [simp]:
     "(c*a = c*b) = (c = (0::'a::division_ring) | a=b)"
by simp
*)
lemma nonzero_imp_inverse_nonzero:
  "a \<noteq> 0 ==> inverse a \<noteq> (0::'a::division_ring)"
proof
  assume ianz: "inverse a = 0"
  assume "a \<noteq> 0"
  hence "1 = a * inverse a" by simp
  also have "... = 0" by (simp add: ianz)
  finally have "1 = (0::'a::division_ring)" .
  thus False by (simp add: eq_commute)
qed


subsection{*Basic Properties of @{term inverse}*}

lemma inverse_zero_imp_zero: "inverse a = 0 ==> a = (0::'a::division_ring)"
apply (rule ccontr) 
apply (blast dest: nonzero_imp_inverse_nonzero) 
done

lemma inverse_nonzero_imp_nonzero:
   "inverse a = 0 ==> a = (0::'a::division_ring)"
apply (rule ccontr) 
apply (blast dest: nonzero_imp_inverse_nonzero) 
done

lemma inverse_nonzero_iff_nonzero [simp]:
   "(inverse a = 0) = (a = (0::'a::{division_ring,division_by_zero}))"
by (force dest: inverse_nonzero_imp_nonzero) 

lemma nonzero_inverse_minus_eq:
      assumes [simp]: "a\<noteq>0"
      shows "inverse(-a) = -inverse(a::'a::division_ring)"
proof -
  have "-a * inverse (- a) = -a * - inverse a"
    by simp
  thus ?thesis 
    by (simp only: mult_cancel_left, simp)
qed

lemma inverse_minus_eq [simp]:
   "inverse(-a) = -inverse(a::'a::{division_ring,division_by_zero})"
proof cases
  assume "a=0" thus ?thesis by (simp add: inverse_zero)
next
  assume "a\<noteq>0" 
  thus ?thesis by (simp add: nonzero_inverse_minus_eq)
qed

lemma nonzero_inverse_eq_imp_eq:
      assumes inveq: "inverse a = inverse b"
	  and anz:  "a \<noteq> 0"
	  and bnz:  "b \<noteq> 0"
	 shows "a = (b::'a::division_ring)"
proof -
  have "a * inverse b = a * inverse a"
    by (simp add: inveq)
  hence "(a * inverse b) * b = (a * inverse a) * b"
    by simp
  thus "a = b"
    by (simp add: mult_assoc anz bnz)
qed

lemma inverse_eq_imp_eq:
  "inverse a = inverse b ==> a = (b::'a::{division_ring,division_by_zero})"
apply (cases "a=0 | b=0") 
 apply (force dest!: inverse_zero_imp_zero
              simp add: eq_commute [of "0::'a"])
apply (force dest!: nonzero_inverse_eq_imp_eq) 
done

lemma inverse_eq_iff_eq [simp]:
  "(inverse a = inverse b) = (a = (b::'a::{division_ring,division_by_zero}))"
by (force dest!: inverse_eq_imp_eq)

lemma nonzero_inverse_inverse_eq:
      assumes [simp]: "a \<noteq> 0"
      shows "inverse(inverse (a::'a::division_ring)) = a"
  proof -
  have "(inverse (inverse a) * inverse a) * a = a" 
    by (simp add: nonzero_imp_inverse_nonzero)
  thus ?thesis
    by (simp add: mult_assoc)
  qed

lemma inverse_inverse_eq [simp]:
     "inverse(inverse (a::'a::{division_ring,division_by_zero})) = a"
  proof cases
    assume "a=0" thus ?thesis by simp
  next
    assume "a\<noteq>0" 
    thus ?thesis by (simp add: nonzero_inverse_inverse_eq)
  qed

lemma inverse_1 [simp]: "inverse 1 = (1::'a::division_ring)"
  proof -
  have "inverse 1 * 1 = (1::'a::division_ring)" 
    by (rule left_inverse [OF zero_neq_one [symmetric]])
  thus ?thesis  by simp
  qed

lemma inverse_unique: 
  assumes ab: "a*b = 1"
  shows "inverse a = (b::'a::division_ring)"
proof -
  have "a \<noteq> 0" using ab by auto
  moreover have "inverse a * (a * b) = inverse a" by (simp add: ab) 
  ultimately show ?thesis by (simp add: mult_assoc [symmetric]) 
qed

lemma nonzero_inverse_mult_distrib: 
      assumes anz: "a \<noteq> 0"
          and bnz: "b \<noteq> 0"
      shows "inverse(a*b) = inverse(b) * inverse(a::'a::division_ring)"
  proof -
  have "inverse(a*b) * (a * b) * inverse(b) = inverse(b)" 
    by (simp add: anz bnz)
  hence "inverse(a*b) * a = inverse(b)" 
    by (simp add: mult_assoc bnz)
  hence "inverse(a*b) * a * inverse(a) = inverse(b) * inverse(a)" 
    by simp
  thus ?thesis
    by (simp add: mult_assoc anz)
  qed

text{*This version builds in division by zero while also re-orienting
      the right-hand side.*}
lemma inverse_mult_distrib [simp]:
     "inverse(a*b) = inverse(a) * inverse(b::'a::{field,division_by_zero})"
  proof cases
    assume "a \<noteq> 0 & b \<noteq> 0" 
    thus ?thesis
      by (simp add: nonzero_inverse_mult_distrib mult_commute)
  next
    assume "~ (a \<noteq> 0 & b \<noteq> 0)" 
    thus ?thesis
      by force
  qed

lemma division_ring_inverse_add:
  "[|(a::'a::division_ring) \<noteq> 0; b \<noteq> 0|]
   ==> inverse a + inverse b = inverse a * (a+b) * inverse b"
by (simp add: ring_simps)

lemma division_ring_inverse_diff:
  "[|(a::'a::division_ring) \<noteq> 0; b \<noteq> 0|]
   ==> inverse a - inverse b = inverse a * (b-a) * inverse b"
by (simp add: ring_simps)

text{*There is no slick version using division by zero.*}
lemma inverse_add:
  "[|a \<noteq> 0;  b \<noteq> 0|]
   ==> inverse a + inverse b = (a+b) * inverse a * inverse (b::'a::field)"
by (simp add: division_ring_inverse_add mult_ac)

lemma inverse_divide [simp]:
  "inverse (a/b) = b / (a::'a::{field,division_by_zero})"
by (simp add: divide_inverse mult_commute)


subsection {* Calculations with fractions *}

text{* There is a whole bunch of simp-rules just for class @{text
field} but none for class @{text field} and @{text nonzero_divides}
because the latter are covered by a simproc. *}

lemma nonzero_mult_divide_mult_cancel_left[simp,noatp]:
assumes [simp]: "b\<noteq>0" and [simp]: "c\<noteq>0" shows "(c*a)/(c*b) = a/(b::'a::field)"
proof -
  have "(c*a)/(c*b) = c * a * (inverse b * inverse c)"
    by (simp add: divide_inverse nonzero_inverse_mult_distrib)
  also have "... =  a * inverse b * (inverse c * c)"
    by (simp only: mult_ac)
  also have "... =  a * inverse b"
    by simp
    finally show ?thesis 
    by (simp add: divide_inverse)
qed

lemma mult_divide_mult_cancel_left:
  "c\<noteq>0 ==> (c*a) / (c*b) = a / (b::'a::{field,division_by_zero})"
apply (cases "b = 0")
apply (simp_all add: nonzero_mult_divide_mult_cancel_left)
done

lemma nonzero_mult_divide_mult_cancel_right [noatp]:
  "[|b\<noteq>0; c\<noteq>0|] ==> (a*c) / (b*c) = a/(b::'a::field)"
by (simp add: mult_commute [of _ c] nonzero_mult_divide_mult_cancel_left) 

lemma mult_divide_mult_cancel_right:
  "c\<noteq>0 ==> (a*c) / (b*c) = a / (b::'a::{field,division_by_zero})"
apply (cases "b = 0")
apply (simp_all add: nonzero_mult_divide_mult_cancel_right)
done

lemma divide_1 [simp]: "a/1 = (a::'a::field)"
by (simp add: divide_inverse)

lemma times_divide_eq_right: "a * (b/c) = (a*b) / (c::'a::field)"
by (simp add: divide_inverse mult_assoc)

lemma times_divide_eq_left: "(b/c) * a = (b*a) / (c::'a::field)"
by (simp add: divide_inverse mult_ac)

lemmas times_divide_eq = times_divide_eq_right times_divide_eq_left

lemma divide_divide_eq_right [simp,noatp]:
  "a / (b/c) = (a*c) / (b::'a::{field,division_by_zero})"
by (simp add: divide_inverse mult_ac)

lemma divide_divide_eq_left [simp,noatp]:
  "(a / b) / (c::'a::{field,division_by_zero}) = a / (b*c)"
by (simp add: divide_inverse mult_assoc)

lemma add_frac_eq: "(y::'a::field) ~= 0 ==> z ~= 0 ==>
    x / y + w / z = (x * z + w * y) / (y * z)"
apply (subgoal_tac "x / y = (x * z) / (y * z)")
apply (erule ssubst)
apply (subgoal_tac "w / z = (w * y) / (y * z)")
apply (erule ssubst)
apply (rule add_divide_distrib [THEN sym])
apply (subst mult_commute)
apply (erule nonzero_mult_divide_mult_cancel_left [THEN sym])
apply assumption
apply (erule nonzero_mult_divide_mult_cancel_right [THEN sym])
apply assumption
done


subsubsection{*Special Cancellation Simprules for Division*}

lemma mult_divide_mult_cancel_left_if[simp,noatp]:
fixes c :: "'a :: {field,division_by_zero}"
shows "(c*a) / (c*b) = (if c=0 then 0 else a/b)"
by (simp add: mult_divide_mult_cancel_left)

lemma nonzero_mult_divide_cancel_right[simp,noatp]:
  "b \<noteq> 0 \<Longrightarrow> a * b / b = (a::'a::field)"
using nonzero_mult_divide_mult_cancel_right[of 1 b a] by simp

lemma nonzero_mult_divide_cancel_left[simp,noatp]:
  "a \<noteq> 0 \<Longrightarrow> a * b / a = (b::'a::field)"
using nonzero_mult_divide_mult_cancel_left[of 1 a b] by simp


lemma nonzero_divide_mult_cancel_right[simp,noatp]:
  "\<lbrakk> a\<noteq>0; b\<noteq>0 \<rbrakk> \<Longrightarrow> b / (a * b) = 1/(a::'a::field)"
using nonzero_mult_divide_mult_cancel_right[of a b 1] by simp

lemma nonzero_divide_mult_cancel_left[simp,noatp]:
  "\<lbrakk> a\<noteq>0; b\<noteq>0 \<rbrakk> \<Longrightarrow> a / (a * b) = 1/(b::'a::field)"
using nonzero_mult_divide_mult_cancel_left[of b a 1] by simp


lemma nonzero_mult_divide_mult_cancel_left2[simp,noatp]:
  "[|b\<noteq>0; c\<noteq>0|] ==> (c*a) / (b*c) = a/(b::'a::field)"
using nonzero_mult_divide_mult_cancel_left[of b c a] by(simp add:mult_ac)

lemma nonzero_mult_divide_mult_cancel_right2[simp,noatp]:
  "[|b\<noteq>0; c\<noteq>0|] ==> (a*c) / (c*b) = a/(b::'a::field)"
using nonzero_mult_divide_mult_cancel_right[of b c a] by(simp add:mult_ac)


subsection {* Division and Unary Minus *}

lemma nonzero_minus_divide_left: "b \<noteq> 0 ==> - (a/b) = (-a) / (b::'a::field)"
by (simp add: divide_inverse minus_mult_left)

lemma nonzero_minus_divide_right: "b \<noteq> 0 ==> - (a/b) = a / -(b::'a::field)"
by (simp add: divide_inverse nonzero_inverse_minus_eq minus_mult_right)

lemma nonzero_minus_divide_divide: "b \<noteq> 0 ==> (-a)/(-b) = a / (b::'a::field)"
by (simp add: divide_inverse nonzero_inverse_minus_eq)

lemma minus_divide_left: "- (a/b) = (-a) / (b::'a::field)"
by (simp add: divide_inverse minus_mult_left [symmetric])

lemma minus_divide_right: "- (a/b) = a / -(b::'a::{field,division_by_zero})"
by (simp add: divide_inverse minus_mult_right [symmetric])


text{*The effect is to extract signs from divisions*}
lemmas divide_minus_left = minus_divide_left [symmetric]
lemmas divide_minus_right = minus_divide_right [symmetric]
declare divide_minus_left [simp]   divide_minus_right [simp]

text{*Also, extract signs from products*}
lemmas mult_minus_left = minus_mult_left [symmetric]
lemmas mult_minus_right = minus_mult_right [symmetric]
declare mult_minus_left [simp]   mult_minus_right [simp]

lemma minus_divide_divide [simp]:
  "(-a)/(-b) = a / (b::'a::{field,division_by_zero})"
apply (cases "b=0", simp) 
apply (simp add: nonzero_minus_divide_divide) 
done

lemma diff_divide_distrib: "(a-b)/(c::'a::field) = a/c - b/c"
by (simp add: diff_minus add_divide_distrib) 

lemma add_divide_eq_iff:
  "(z::'a::field) \<noteq> 0 \<Longrightarrow> x + y/z = (z*x + y)/z"
by(simp add:add_divide_distrib nonzero_mult_divide_cancel_left)

lemma divide_add_eq_iff:
  "(z::'a::field) \<noteq> 0 \<Longrightarrow> x/z + y = (x + z*y)/z"
by(simp add:add_divide_distrib nonzero_mult_divide_cancel_left)

lemma diff_divide_eq_iff:
  "(z::'a::field) \<noteq> 0 \<Longrightarrow> x - y/z = (z*x - y)/z"
by(simp add:diff_divide_distrib nonzero_mult_divide_cancel_left)

lemma divide_diff_eq_iff:
  "(z::'a::field) \<noteq> 0 \<Longrightarrow> x/z - y = (x - z*y)/z"
by(simp add:diff_divide_distrib nonzero_mult_divide_cancel_left)

lemma nonzero_eq_divide_eq: "c\<noteq>0 ==> ((a::'a::field) = b/c) = (a*c = b)"
proof -
  assume [simp]: "c\<noteq>0"
  have "(a = b/c) = (a*c = (b/c)*c)" by simp
  also have "... = (a*c = b)" by (simp add: divide_inverse mult_assoc)
  finally show ?thesis .
qed

lemma nonzero_divide_eq_eq: "c\<noteq>0 ==> (b/c = (a::'a::field)) = (b = a*c)"
proof -
  assume [simp]: "c\<noteq>0"
  have "(b/c = a) = ((b/c)*c = a*c)"  by simp
  also have "... = (b = a*c)"  by (simp add: divide_inverse mult_assoc) 
  finally show ?thesis .
qed

lemma eq_divide_eq:
  "((a::'a::{field,division_by_zero}) = b/c) = (if c\<noteq>0 then a*c = b else a=0)"
by (simp add: nonzero_eq_divide_eq) 

lemma divide_eq_eq:
  "(b/c = (a::'a::{field,division_by_zero})) = (if c\<noteq>0 then b = a*c else a=0)"
by (force simp add: nonzero_divide_eq_eq) 

lemma divide_eq_imp: "(c::'a::{division_by_zero,field}) ~= 0 ==>
    b = a * c ==> b / c = a"
  by (subst divide_eq_eq, simp)

lemma eq_divide_imp: "(c::'a::{division_by_zero,field}) ~= 0 ==>
    a * c = b ==> a = b / c"
  by (subst eq_divide_eq, simp)


lemmas field_eq_simps = ring_simps
  (* pull / out*)
  add_divide_eq_iff divide_add_eq_iff
  diff_divide_eq_iff divide_diff_eq_iff
  (* multiply eqn *)
  nonzero_eq_divide_eq nonzero_divide_eq_eq
(* is added later:
  times_divide_eq_left times_divide_eq_right
*)

text{*An example:*}
lemma fixes a b c d e f :: "'a::field"
shows "\<lbrakk>a\<noteq>b; c\<noteq>d; e\<noteq>f \<rbrakk> \<Longrightarrow> ((a-b)*(c-d)*(e-f))/((c-d)*(e-f)*(a-b)) = 1"
apply(subgoal_tac "(c-d)*(e-f)*(a-b) \<noteq> 0")
 apply(simp add:field_eq_simps)
apply(simp)
done


lemma diff_frac_eq: "(y::'a::field) ~= 0 ==> z ~= 0 ==>
    x / y - w / z = (x * z - w * y) / (y * z)"
by (simp add:field_eq_simps times_divide_eq)

lemma frac_eq_eq: "(y::'a::field) ~= 0 ==> z ~= 0 ==>
    (x / y = w / z) = (x * z = w * y)"
by (simp add:field_eq_simps times_divide_eq)


subsection {* Ordered Fields *}

lemma positive_imp_inverse_positive: 
assumes a_gt_0: "0 < a"  shows "0 < inverse (a::'a::ordered_field)"
proof -
  have "0 < a * inverse a" 
    by (simp add: a_gt_0 [THEN order_less_imp_not_eq2] zero_less_one)
  thus "0 < inverse a" 
    by (simp add: a_gt_0 [THEN order_less_not_sym] zero_less_mult_iff)
qed

lemma negative_imp_inverse_negative:
  "a < 0 ==> inverse a < (0::'a::ordered_field)"
by (insert positive_imp_inverse_positive [of "-a"], 
    simp add: nonzero_inverse_minus_eq order_less_imp_not_eq)

lemma inverse_le_imp_le:
assumes invle: "inverse a \<le> inverse b" and apos:  "0 < a"
shows "b \<le> (a::'a::ordered_field)"
proof (rule classical)
  assume "~ b \<le> a"
  hence "a < b"  by (simp add: linorder_not_le)
  hence bpos: "0 < b"  by (blast intro: apos order_less_trans)
  hence "a * inverse a \<le> a * inverse b"
    by (simp add: apos invle order_less_imp_le mult_left_mono)
  hence "(a * inverse a) * b \<le> (a * inverse b) * b"
    by (simp add: bpos order_less_imp_le mult_right_mono)
  thus "b \<le> a"  by (simp add: mult_assoc apos bpos order_less_imp_not_eq2)
qed

lemma inverse_positive_imp_positive:
assumes inv_gt_0: "0 < inverse a" and nz: "a \<noteq> 0"
shows "0 < (a::'a::ordered_field)"
proof -
  have "0 < inverse (inverse a)"
    using inv_gt_0 by (rule positive_imp_inverse_positive)
  thus "0 < a"
    using nz by (simp add: nonzero_inverse_inverse_eq)
qed

lemma inverse_positive_iff_positive [simp]: