src/HOL/Fields.thy
 author haftmann Wed, 10 Feb 2010 14:12:02 +0100 changeset 35090 88cc65ae046e parent 35084 e25eedfc15ce child 35216 7641e8d831d2 permissions -rw-r--r--
moved lemma field_le_epsilon from Real.thy to Fields.thy
```
(*  Title:      HOL/Fields.thy
Author:     Gertrud Bauer
Author:     Steven Obua
Author:     Tobias Nipkow
Author:     Lawrence C Paulson
Author:     Markus Wenzel
*)

theory Fields
imports Rings
begin

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

subclass 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)
next
fix a b :: 'a
show "a / b = a * inverse b" by (rule field_divide_inverse)
qed

subclass idom ..

lemma right_inverse_eq: "b \<noteq> 0 \<Longrightarrow> a / b = 1 \<longleftrightarrow> a = b"
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 \<Longrightarrow> inverse a = 1 / a"

lemma divide_self [simp]: "a \<noteq> 0 \<Longrightarrow> a / a = 1"

lemma divide_zero_left [simp]: "0 / a = 0"

lemma inverse_eq_divide: "inverse a = 1 / a"

lemma add_divide_distrib: "(a+b) / c = a/c + b/c"

text{*There is no slick version using division by zero.*}
"[| a \<noteq> 0;  b \<noteq> 0 |]
==> inverse a + inverse b = (a + b) * inverse a * inverse b"

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"
proof -
have "(c*a)/(c*b) = c * a * (inverse b * inverse c)"
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 nonzero_mult_divide_mult_cancel_right [simp, noatp]:
"\<lbrakk>b \<noteq> 0; c \<noteq> 0\<rbrakk> \<Longrightarrow> (a * c) / (b * c) = a / b"
by (simp add: mult_commute [of _ c])

lemma divide_1 [simp]: "a / 1 = a"

lemma times_divide_eq_right: "a * (b / c) = (a * b) / c"

lemma times_divide_eq_left: "(b / c) * a = (b * a) / c"

text {* These are later declared as simp rules. *}
lemmas times_divide_eq [noatp] = times_divide_eq_right times_divide_eq_left

assumes "y \<noteq> 0" and "z \<noteq> 0"
shows "x / y + w / z = (x * z + w * y) / (y * z)"
proof -
have "x / y + w / z = (x * z) / (y * z) + (y * w) / (y * z)"
using assms by simp
also have "\<dots> = (x * z + y * w) / (y * z)"
finally show ?thesis
by (simp only: mult_commute)
qed

text{*Special Cancellation Simprules for Division*}

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

lemma nonzero_mult_divide_mult_cancel_left2 [simp, noatp]:
"\<lbrakk>b \<noteq> 0; c \<noteq> 0\<rbrakk> \<Longrightarrow> (c * a) / (b * c) = a / b"
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]:
"\<lbrakk>b \<noteq> 0; c \<noteq> 0\<rbrakk> \<Longrightarrow> (a * c) / (c * b) = a / b"
using nonzero_mult_divide_mult_cancel_right [of b c a] by (simp add: mult_ac)

lemma minus_divide_left: "- (a / b) = (-a) / b"

lemma nonzero_minus_divide_right: "b \<noteq> 0 ==> - (a / b) = a / (- b)"

lemma nonzero_minus_divide_divide: "b \<noteq> 0 ==> (-a) / (-b) = a / b"

lemma divide_minus_left [simp, noatp]: "(-a) / b = - (a / b)"

lemma diff_divide_distrib: "(a - b) / c = a / c - b / c"

"z \<noteq> 0 \<Longrightarrow> x + y / z = (z * x + y) / z"

"z \<noteq> 0 \<Longrightarrow> x / z + y = (x + z * y) / z"

lemma diff_divide_eq_iff:
"z \<noteq> 0 \<Longrightarrow> x - y / z = (z * x - y) / z"

lemma divide_diff_eq_iff:
"z \<noteq> 0 \<Longrightarrow> x / z - y = (x - z * y) / z"

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

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

lemma divide_eq_imp: "c \<noteq> 0 \<Longrightarrow> b = a * c \<Longrightarrow> b / c = a"
by simp

lemma eq_divide_imp: "c \<noteq> 0 \<Longrightarrow> a * c = b \<Longrightarrow> a = b / c"
by (erule subst, simp)

lemmas field_eq_simps[noatp] = algebra_simps
(* pull / out*)
diff_divide_eq_iff divide_diff_eq_iff
(* multiply eqn *)
nonzero_eq_divide_eq nonzero_divide_eq_eq
times_divide_eq_left times_divide_eq_right
*)

text{*An example:*}
lemma "\<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)
done

lemma diff_frac_eq:
"y \<noteq> 0 \<Longrightarrow> z \<noteq> 0 \<Longrightarrow> x / y - w / z = (x * z - w * y) / (y * z)"

lemma frac_eq_eq:
"y \<noteq> 0 \<Longrightarrow> z \<noteq> 0 \<Longrightarrow> (x / y = w / z) = (x * z = w * y)"

end

class division_by_zero = zero + inverse +
assumes inverse_zero [simp]: "inverse 0 = 0"

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

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

class linordered_field = field + linordered_idom

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

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 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
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 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

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 inverse_divide [simp]:
"inverse (a/b) = b / (a::'a::{field,division_by_zero})"

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

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

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

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

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)"

subsection {* Division and Unary Minus *}

lemma minus_divide_right: "- (a/b) = a / -(b::'a::{field,division_by_zero})"

lemma divide_minus_right [simp, noatp]:
"a / -(b::'a::{field,division_by_zero}) = -(a / b)"

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

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

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

subsection {* Ordered Fields *}

lemma positive_imp_inverse_positive:
assumes a_gt_0: "0 < a"  shows "0 < inverse (a::'a::linordered_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::linordered_field)"
by (insert positive_imp_inverse_positive [of "-a"],

lemma inverse_le_imp_le:
assumes invle: "inverse a \<le> inverse b" and apos:  "0 < a"
shows "b \<le> (a::'a::linordered_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::linordered_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]:
"(0 < inverse a) = (0 < (a::'a::{linordered_field,division_by_zero}))"
apply (cases "a = 0", simp)
apply (blast intro: inverse_positive_imp_positive positive_imp_inverse_positive)
done

lemma inverse_negative_imp_negative:
assumes inv_less_0: "inverse a < 0" and nz:  "a \<noteq> 0"
shows "a < (0::'a::linordered_field)"
proof -
have "inverse (inverse a) < 0"
using inv_less_0 by (rule negative_imp_inverse_negative)
thus "a < 0" using nz by (simp add: nonzero_inverse_inverse_eq)
qed

lemma inverse_negative_iff_negative [simp]:
"(inverse a < 0) = (a < (0::'a::{linordered_field,division_by_zero}))"
apply (cases "a = 0", simp)
apply (blast intro: inverse_negative_imp_negative negative_imp_inverse_negative)
done

lemma inverse_nonnegative_iff_nonnegative [simp]:
"(0 \<le> inverse a) = (0 \<le> (a::'a::{linordered_field,division_by_zero}))"

lemma inverse_nonpositive_iff_nonpositive [simp]:
"(inverse a \<le> 0) = (a \<le> (0::'a::{linordered_field,division_by_zero}))"

lemma linordered_field_no_lb: "\<forall> x. \<exists>y. y < (x::'a::linordered_field)"
proof
fix x::'a
have m1: "- (1::'a) < 0" by simp
have "(- 1) + x < x" by simp
thus "\<exists>y. y < x" by blast
qed

lemma linordered_field_no_ub: "\<forall> x. \<exists>y. y > (x::'a::linordered_field)"
proof
fix x::'a
have m1: " (1::'a) > 0" by simp
have "1 + x > x" by simp
thus "\<exists>y. y > x" by blast
qed

subsection{*Anti-Monotonicity of @{term inverse}*}

lemma less_imp_inverse_less:
assumes less: "a < b" and apos:  "0 < a"
shows "inverse b < inverse (a::'a::linordered_field)"
proof (rule ccontr)
assume "~ inverse b < inverse a"
hence "inverse a \<le> inverse b" by (simp add: linorder_not_less)
hence "~ (a < b)"
by (simp add: linorder_not_less inverse_le_imp_le [OF _ apos])
thus False by (rule notE [OF _ less])
qed

lemma inverse_less_imp_less:
"[|inverse a < inverse b; 0 < a|] ==> b < (a::'a::linordered_field)"
apply (simp add: order_less_le [of "inverse a"] order_less_le [of "b"])
apply (force dest!: inverse_le_imp_le nonzero_inverse_eq_imp_eq)
done

text{*Both premises are essential. Consider -1 and 1.*}
lemma inverse_less_iff_less [simp,noatp]:
"[|0 < a; 0 < b|] ==> (inverse a < inverse b) = (b < (a::'a::linordered_field))"
by (blast intro: less_imp_inverse_less dest: inverse_less_imp_less)

lemma le_imp_inverse_le:
"[|a \<le> b; 0 < a|] ==> inverse b \<le> inverse (a::'a::linordered_field)"
by (force simp add: order_le_less less_imp_inverse_less)

lemma inverse_le_iff_le [simp,noatp]:
"[|0 < a; 0 < b|] ==> (inverse a \<le> inverse b) = (b \<le> (a::'a::linordered_field))"
by (blast intro: le_imp_inverse_le dest: inverse_le_imp_le)

text{*These results refer to both operands being negative.  The opposite-sign
case is trivial, since inverse preserves signs.*}
lemma inverse_le_imp_le_neg:
"[|inverse a \<le> inverse b; b < 0|] ==> b \<le> (a::'a::linordered_field)"
apply (rule classical)
apply (subgoal_tac "a < 0")
prefer 2 apply (force simp add: linorder_not_le intro: order_less_trans)
apply (insert inverse_le_imp_le [of "-b" "-a"])
done

lemma less_imp_inverse_less_neg:
"[|a < b; b < 0|] ==> inverse b < inverse (a::'a::linordered_field)"
apply (subgoal_tac "a < 0")
prefer 2 apply (blast intro: order_less_trans)
apply (insert less_imp_inverse_less [of "-b" "-a"])
done

lemma inverse_less_imp_less_neg:
"[|inverse a < inverse b; b < 0|] ==> b < (a::'a::linordered_field)"
apply (rule classical)
apply (subgoal_tac "a < 0")
prefer 2
apply (force simp add: linorder_not_less intro: order_le_less_trans)
apply (insert inverse_less_imp_less [of "-b" "-a"])
done

lemma inverse_less_iff_less_neg [simp,noatp]:
"[|a < 0; b < 0|] ==> (inverse a < inverse b) = (b < (a::'a::linordered_field))"
apply (insert inverse_less_iff_less [of "-b" "-a"])
apply (simp del: inverse_less_iff_less
done

lemma le_imp_inverse_le_neg:
"[|a \<le> b; b < 0|] ==> inverse b \<le> inverse (a::'a::linordered_field)"
by (force simp add: order_le_less less_imp_inverse_less_neg)

lemma inverse_le_iff_le_neg [simp,noatp]:
"[|a < 0; b < 0|] ==> (inverse a \<le> inverse b) = (b \<le> (a::'a::linordered_field))"
by (blast intro: le_imp_inverse_le_neg dest: inverse_le_imp_le_neg)

subsection{*Inverses and the Number One*}

lemma one_less_inverse_iff:
"(1 < inverse x) = (0 < x & x < (1::'a::{linordered_field,division_by_zero}))"
proof cases
assume "0 < x"
with inverse_less_iff_less [OF zero_less_one, of x]
show ?thesis by simp
next
assume notless: "~ (0 < x)"
have "~ (1 < inverse x)"
proof
assume "1 < inverse x"
also with notless have "... \<le> 0" by (simp add: linorder_not_less)
also have "... < 1" by (rule zero_less_one)
finally show False by auto
qed
with notless show ?thesis by simp
qed

lemma inverse_eq_1_iff [simp]:
"(inverse x = 1) = (x = (1::'a::{field,division_by_zero}))"
by (insert inverse_eq_iff_eq [of x 1], simp)

lemma one_le_inverse_iff:
"(1 \<le> inverse x) = (0 < x & x \<le> (1::'a::{linordered_field,division_by_zero}))"
by (force simp add: order_le_less one_less_inverse_iff zero_less_one
eq_commute [of 1])

lemma inverse_less_1_iff:
"(inverse x < 1) = (x \<le> 0 | 1 < (x::'a::{linordered_field,division_by_zero}))"
by (simp add: linorder_not_le [symmetric] one_le_inverse_iff)

lemma inverse_le_1_iff:
"(inverse x \<le> 1) = (x \<le> 0 | 1 \<le> (x::'a::{linordered_field,division_by_zero}))"
by (simp add: linorder_not_less [symmetric] one_less_inverse_iff)

subsection{*Simplification of Inequalities Involving Literal Divisors*}

lemma pos_le_divide_eq: "0 < (c::'a::linordered_field) ==> (a \<le> b/c) = (a*c \<le> b)"
proof -
assume less: "0<c"
hence "(a \<le> b/c) = (a*c \<le> (b/c)*c)"
by (simp add: mult_le_cancel_right order_less_not_sym [OF less])
also have "... = (a*c \<le> b)"
by (simp add: order_less_imp_not_eq2 [OF less] divide_inverse mult_assoc)
finally show ?thesis .
qed

lemma neg_le_divide_eq: "c < (0::'a::linordered_field) ==> (a \<le> b/c) = (b \<le> a*c)"
proof -
assume less: "c<0"
hence "(a \<le> b/c) = ((b/c)*c \<le> a*c)"
by (simp add: mult_le_cancel_right order_less_not_sym [OF less])
also have "... = (b \<le> a*c)"
by (simp add: order_less_imp_not_eq [OF less] divide_inverse mult_assoc)
finally show ?thesis .
qed

lemma le_divide_eq:
"(a \<le> b/c) =
(if 0 < c then a*c \<le> b
else if c < 0 then b \<le> a*c
else  a \<le> (0::'a::{linordered_field,division_by_zero}))"
apply (cases "c=0", simp)
apply (force simp add: pos_le_divide_eq neg_le_divide_eq linorder_neq_iff)
done

lemma pos_divide_le_eq: "0 < (c::'a::linordered_field) ==> (b/c \<le> a) = (b \<le> a*c)"
proof -
assume less: "0<c"
hence "(b/c \<le> a) = ((b/c)*c \<le> a*c)"
by (simp add: mult_le_cancel_right order_less_not_sym [OF less])
also have "... = (b \<le> a*c)"
by (simp add: order_less_imp_not_eq2 [OF less] divide_inverse mult_assoc)
finally show ?thesis .
qed

lemma neg_divide_le_eq: "c < (0::'a::linordered_field) ==> (b/c \<le> a) = (a*c \<le> b)"
proof -
assume less: "c<0"
hence "(b/c \<le> a) = (a*c \<le> (b/c)*c)"
by (simp add: mult_le_cancel_right order_less_not_sym [OF less])
also have "... = (a*c \<le> b)"
by (simp add: order_less_imp_not_eq [OF less] divide_inverse mult_assoc)
finally show ?thesis .
qed

lemma divide_le_eq:
"(b/c \<le> a) =
(if 0 < c then b \<le> a*c
else if c < 0 then a*c \<le> b
else 0 \<le> (a::'a::{linordered_field,division_by_zero}))"
apply (cases "c=0", simp)
apply (force simp add: pos_divide_le_eq neg_divide_le_eq linorder_neq_iff)
done

lemma pos_less_divide_eq:
"0 < (c::'a::linordered_field) ==> (a < b/c) = (a*c < b)"
proof -
assume less: "0<c"
hence "(a < b/c) = (a*c < (b/c)*c)"
by (simp add: mult_less_cancel_right_disj order_less_not_sym [OF less])
also have "... = (a*c < b)"
by (simp add: order_less_imp_not_eq2 [OF less] divide_inverse mult_assoc)
finally show ?thesis .
qed

lemma neg_less_divide_eq:
"c < (0::'a::linordered_field) ==> (a < b/c) = (b < a*c)"
proof -
assume less: "c<0"
hence "(a < b/c) = ((b/c)*c < a*c)"
by (simp add: mult_less_cancel_right_disj order_less_not_sym [OF less])
also have "... = (b < a*c)"
by (simp add: order_less_imp_not_eq [OF less] divide_inverse mult_assoc)
finally show ?thesis .
qed

lemma less_divide_eq:
"(a < b/c) =
(if 0 < c then a*c < b
else if c < 0 then b < a*c
else  a < (0::'a::{linordered_field,division_by_zero}))"
apply (cases "c=0", simp)
apply (force simp add: pos_less_divide_eq neg_less_divide_eq linorder_neq_iff)
done

lemma pos_divide_less_eq:
"0 < (c::'a::linordered_field) ==> (b/c < a) = (b < a*c)"
proof -
assume less: "0<c"
hence "(b/c < a) = ((b/c)*c < a*c)"
by (simp add: mult_less_cancel_right_disj order_less_not_sym [OF less])
also have "... = (b < a*c)"
by (simp add: order_less_imp_not_eq2 [OF less] divide_inverse mult_assoc)
finally show ?thesis .
qed

lemma neg_divide_less_eq:
"c < (0::'a::linordered_field) ==> (b/c < a) = (a*c < b)"
proof -
assume less: "c<0"
hence "(b/c < a) = (a*c < (b/c)*c)"
by (simp add: mult_less_cancel_right_disj order_less_not_sym [OF less])
also have "... = (a*c < b)"
by (simp add: order_less_imp_not_eq [OF less] divide_inverse mult_assoc)
finally show ?thesis .
qed

lemma divide_less_eq:
"(b/c < a) =
(if 0 < c then b < a*c
else if c < 0 then a*c < b
else 0 < (a::'a::{linordered_field,division_by_zero}))"
apply (cases "c=0", simp)
apply (force simp add: pos_divide_less_eq neg_divide_less_eq linorder_neq_iff)
done

subsection{*Field simplification*}

text{* Lemmas @{text field_simps} multiply with denominators in in(equations)
if they can be proved to be non-zero (for equations) or positive/negative
(for inequations). Can be too aggressive and is therefore separate from the
more benign @{text algebra_simps}. *}

lemmas field_simps[noatp] = field_eq_simps
(* multiply ineqn *)
pos_divide_less_eq neg_divide_less_eq
pos_less_divide_eq neg_less_divide_eq
pos_divide_le_eq neg_divide_le_eq
pos_le_divide_eq neg_le_divide_eq

text{* Lemmas @{text sign_simps} is a first attempt to automate proofs
of positivity/negativity needed for @{text field_simps}. Have not added @{text
sign_simps} to @{text field_simps} because the former can lead to case
explosions. *}

lemmas sign_simps[noatp] = group_simps
zero_less_mult_iff  mult_less_0_iff

(* Only works once linear arithmetic is installed:
text{*An example:*}
lemma fixes a b c d e f :: "'a::linordered_field"
shows "\<lbrakk>a>b; c<d; e<f; 0 < u \<rbrakk> \<Longrightarrow>
((a-b)*(c-d)*(e-f))/((c-d)*(e-f)*(a-b)) <
((e-f)*(a-b)*(c-d))/((e-f)*(a-b)*(c-d)) + u"
apply(subgoal_tac "(c-d)*(e-f)*(a-b) > 0")
apply(subgoal_tac "(c-d)*(e-f)*(a-b)*u > 0")
done
*)

subsection{*Division and Signs*}

lemma zero_less_divide_iff:
"((0::'a::{linordered_field,division_by_zero}) < a/b) = (0 < a & 0 < b | a < 0 & b < 0)"

lemma divide_less_0_iff:
"(a/b < (0::'a::{linordered_field,division_by_zero})) =
(0 < a & b < 0 | a < 0 & 0 < b)"

lemma zero_le_divide_iff:
"((0::'a::{linordered_field,division_by_zero}) \<le> a/b) =
(0 \<le> a & 0 \<le> b | a \<le> 0 & b \<le> 0)"

lemma divide_le_0_iff:
"(a/b \<le> (0::'a::{linordered_field,division_by_zero})) =
(0 \<le> a & b \<le> 0 | a \<le> 0 & 0 \<le> b)"

lemma divide_eq_0_iff [simp,noatp]:
"(a/b = 0) = (a=0 | b=(0::'a::{field,division_by_zero}))"

lemma divide_pos_pos:
"0 < (x::'a::linordered_field) ==> 0 < y ==> 0 < x / y"

lemma divide_nonneg_pos:
"0 <= (x::'a::linordered_field) ==> 0 < y ==> 0 <= x / y"

lemma divide_neg_pos:
"(x::'a::linordered_field) < 0 ==> 0 < y ==> x / y < 0"

lemma divide_nonpos_pos:
"(x::'a::linordered_field) <= 0 ==> 0 < y ==> x / y <= 0"

lemma divide_pos_neg:
"0 < (x::'a::linordered_field) ==> y < 0 ==> x / y < 0"

lemma divide_nonneg_neg:
"0 <= (x::'a::linordered_field) ==> y < 0 ==> x / y <= 0"

lemma divide_neg_neg:
"(x::'a::linordered_field) < 0 ==> y < 0 ==> 0 < x / y"

lemma divide_nonpos_neg:
"(x::'a::linordered_field) <= 0 ==> y < 0 ==> 0 <= x / y"

subsection{*Cancellation Laws for Division*}

lemma divide_cancel_right [simp,noatp]:
"(a/c = b/c) = (c = 0 | a = (b::'a::{field,division_by_zero}))"
apply (cases "c=0", simp)
done

lemma divide_cancel_left [simp,noatp]:
"(c/a = c/b) = (c = 0 | a = (b::'a::{field,division_by_zero}))"
apply (cases "c=0", simp)
done

subsection {* Division and the Number One *}

text{*Simplify expressions equated with 1*}
lemma divide_eq_1_iff [simp,noatp]:
"(a/b = 1) = (b \<noteq> 0 & a = (b::'a::{field,division_by_zero}))"
apply (cases "b=0", simp)
done

lemma one_eq_divide_iff [simp,noatp]:
"(1 = a/b) = (b \<noteq> 0 & a = (b::'a::{field,division_by_zero}))"
by (simp add: eq_commute [of 1])

lemma zero_eq_1_divide_iff [simp,noatp]:
"((0::'a::{linordered_field,division_by_zero}) = 1/a) = (a = 0)"
apply (cases "a=0", simp)
done

lemma one_divide_eq_0_iff [simp,noatp]:
"(1/a = (0::'a::{linordered_field,division_by_zero})) = (a = 0)"
apply (cases "a=0", simp)
apply (insert zero_neq_one [THEN not_sym])
done

text{*Simplify expressions such as @{text "0 < 1/x"} to @{text "0 < x"}*}
lemmas zero_less_divide_1_iff = zero_less_divide_iff [of 1, simplified]
lemmas divide_less_0_1_iff = divide_less_0_iff [of 1, simplified]
lemmas zero_le_divide_1_iff = zero_le_divide_iff [of 1, simplified]
lemmas divide_le_0_1_iff = divide_le_0_iff [of 1, simplified]

declare zero_less_divide_1_iff [simp,noatp]
declare divide_less_0_1_iff [simp,noatp]
declare zero_le_divide_1_iff [simp,noatp]
declare divide_le_0_1_iff [simp,noatp]

subsection {* Ordering Rules for Division *}

lemma divide_strict_right_mono:
"[|a < b; 0 < c|] ==> a / c < b / (c::'a::linordered_field)"
by (simp add: order_less_imp_not_eq2 divide_inverse mult_strict_right_mono
positive_imp_inverse_positive)

lemma divide_right_mono:
"[|a \<le> b; 0 \<le> c|] ==> a/c \<le> b/(c::'a::{linordered_field,division_by_zero})"
by (force simp add: divide_strict_right_mono order_le_less)

lemma divide_right_mono_neg: "(a::'a::{division_by_zero,linordered_field}) <= b
==> c <= 0 ==> b / c <= a / c"
apply (drule divide_right_mono [of _ _ "- c"])
apply auto
done

lemma divide_strict_right_mono_neg:
"[|b < a; c < 0|] ==> a / c < b / (c::'a::linordered_field)"
apply (drule divide_strict_right_mono [of _ _ "-c"], simp)
apply (simp add: order_less_imp_not_eq nonzero_minus_divide_right [symmetric])
done

text{*The last premise ensures that @{term a} and @{term b}
have the same sign*}
lemma divide_strict_left_mono:
"[|b < a; 0 < c; 0 < a*b|] ==> c / a < c / (b::'a::linordered_field)"
by(auto simp: field_simps times_divide_eq zero_less_mult_iff mult_strict_right_mono)

lemma divide_left_mono:
"[|b \<le> a; 0 \<le> c; 0 < a*b|] ==> c / a \<le> c / (b::'a::linordered_field)"
by(auto simp: field_simps times_divide_eq zero_less_mult_iff mult_right_mono)

lemma divide_left_mono_neg: "(a::'a::{division_by_zero,linordered_field}) <= b
==> c <= 0 ==> 0 < a * b ==> c / a <= c / b"
apply (drule divide_left_mono [of _ _ "- c"])
done

lemma divide_strict_left_mono_neg:
"[|a < b; c < 0; 0 < a*b|] ==> c / a < c / (b::'a::linordered_field)"
by(auto simp: field_simps times_divide_eq zero_less_mult_iff mult_strict_right_mono_neg)

text{*Simplify quotients that are compared with the value 1.*}

lemma le_divide_eq_1 [noatp]:
fixes a :: "'a :: {linordered_field,division_by_zero}"
shows "(1 \<le> b / a) = ((0 < a & a \<le> b) | (a < 0 & b \<le> a))"

lemma divide_le_eq_1 [noatp]:
fixes a :: "'a :: {linordered_field,division_by_zero}"
shows "(b / a \<le> 1) = ((0 < a & b \<le> a) | (a < 0 & a \<le> b) | a=0)"

lemma less_divide_eq_1 [noatp]:
fixes a :: "'a :: {linordered_field,division_by_zero}"
shows "(1 < b / a) = ((0 < a & a < b) | (a < 0 & b < a))"

lemma divide_less_eq_1 [noatp]:
fixes a :: "'a :: {linordered_field,division_by_zero}"
shows "(b / a < 1) = ((0 < a & b < a) | (a < 0 & a < b) | a=0)"

subsection{*Conditional Simplification Rules: No Case Splits*}

lemma le_divide_eq_1_pos [simp,noatp]:
fixes a :: "'a :: {linordered_field,division_by_zero}"
shows "0 < a \<Longrightarrow> (1 \<le> b/a) = (a \<le> b)"

lemma le_divide_eq_1_neg [simp,noatp]:
fixes a :: "'a :: {linordered_field,division_by_zero}"
shows "a < 0 \<Longrightarrow> (1 \<le> b/a) = (b \<le> a)"

lemma divide_le_eq_1_pos [simp,noatp]:
fixes a :: "'a :: {linordered_field,division_by_zero}"
shows "0 < a \<Longrightarrow> (b/a \<le> 1) = (b \<le> a)"

lemma divide_le_eq_1_neg [simp,noatp]:
fixes a :: "'a :: {linordered_field,division_by_zero}"
shows "a < 0 \<Longrightarrow> (b/a \<le> 1) = (a \<le> b)"

lemma less_divide_eq_1_pos [simp,noatp]:
fixes a :: "'a :: {linordered_field,division_by_zero}"
shows "0 < a \<Longrightarrow> (1 < b/a) = (a < b)"

lemma less_divide_eq_1_neg [simp,noatp]:
fixes a :: "'a :: {linordered_field,division_by_zero}"
shows "a < 0 \<Longrightarrow> (1 < b/a) = (b < a)"

lemma divide_less_eq_1_pos [simp,noatp]:
fixes a :: "'a :: {linordered_field,division_by_zero}"
shows "0 < a \<Longrightarrow> (b/a < 1) = (b < a)"

lemma divide_less_eq_1_neg [simp,noatp]:
fixes a :: "'a :: {linordered_field,division_by_zero}"
shows "a < 0 \<Longrightarrow> b/a < 1 <-> a < b"

lemma eq_divide_eq_1 [simp,noatp]:
fixes a :: "'a :: {linordered_field,division_by_zero}"
shows "(1 = b/a) = ((a \<noteq> 0 & a = b))"

lemma divide_eq_eq_1 [simp,noatp]:
fixes a :: "'a :: {linordered_field,division_by_zero}"
shows "(b/a = 1) = ((a \<noteq> 0 & a = b))"

subsection {* Reasoning about inequalities with division *}

lemma mult_imp_div_pos_le: "0 < (y::'a::linordered_field) ==> x <= z * y ==>
x / y <= z"
by (subst pos_divide_le_eq, assumption+)

lemma mult_imp_le_div_pos: "0 < (y::'a::linordered_field) ==> z * y <= x ==>
z <= x / y"

lemma mult_imp_div_pos_less: "0 < (y::'a::linordered_field) ==> x < z * y ==>
x / y < z"

lemma mult_imp_less_div_pos: "0 < (y::'a::linordered_field) ==> z * y < x ==>
z < x / y"

lemma frac_le: "(0::'a::linordered_field) <= x ==>
x <= y ==> 0 < w ==> w <= z  ==> x / z <= y / w"
apply (rule mult_imp_div_pos_le)
apply simp
apply (subst times_divide_eq_left)
apply (rule mult_imp_le_div_pos, assumption)
apply (rule mult_mono)
apply simp_all
done

lemma frac_less: "(0::'a::linordered_field) <= x ==>
x < y ==> 0 < w ==> w <= z  ==> x / z < y / w"
apply (rule mult_imp_div_pos_less)
apply simp
apply (subst times_divide_eq_left)
apply (rule mult_imp_less_div_pos, assumption)
apply (erule mult_less_le_imp_less)
apply simp_all
done

lemma frac_less2: "(0::'a::linordered_field) < x ==>
x <= y ==> 0 < w ==> w < z  ==> x / z < y / w"
apply (rule mult_imp_div_pos_less)
apply simp_all
apply (subst times_divide_eq_left)
apply (rule mult_imp_less_div_pos, assumption)
apply (erule mult_le_less_imp_less)
apply simp_all
done

text{*It's not obvious whether these should be simprules or not.
Their effect is to gather terms into one big fraction, like
a*b*c / x*y*z. The rationale for that is unclear, but many proofs
seem to need them.*}

declare times_divide_eq [simp]

subsection {* Ordered Fields are Dense *}

lemma less_half_sum: "a < b ==> a < (a+b) / (1+1::'a::linordered_field)"

lemma gt_half_sum: "a < b ==> (a+b)/(1+1::'a::linordered_field) < b"

instance linordered_field < dense_linorder
proof
fix x y :: 'a
have "x < x + 1" by simp
then show "\<exists>y. x < y" ..
have "x - 1 < x" by simp
then show "\<exists>y. y < x" ..
show "x < y \<Longrightarrow> \<exists>z>x. z < y" by (blast intro!: less_half_sum gt_half_sum)
qed

subsection {* Absolute Value *}

lemma nonzero_abs_inverse:
"a \<noteq> 0 ==> abs (inverse (a::'a::linordered_field)) = inverse (abs a)"
apply (auto simp add: linorder_neq_iff abs_if nonzero_inverse_minus_eq
negative_imp_inverse_negative)
apply (blast intro: positive_imp_inverse_positive elim: order_less_asym)
done

lemma abs_inverse [simp]:
"abs (inverse (a::'a::{linordered_field,division_by_zero})) =
inverse (abs a)"
apply (cases "a=0", simp)
done

lemma nonzero_abs_divide:
"b \<noteq> 0 ==> abs (a / (b::'a::linordered_field)) = abs a / abs b"
by (simp add: divide_inverse abs_mult nonzero_abs_inverse)

lemma abs_divide [simp]:
"abs (a / (b::'a::{linordered_field,division_by_zero})) = abs a / abs b"
apply (cases "b=0", simp)
done

lemma abs_div_pos: "(0::'a::{division_by_zero,linordered_field}) < y ==>
abs x / y = abs (x / y)"
apply (subst abs_divide)
done

lemma field_le_epsilon:
fixes x y :: "'a :: {division_by_zero,linordered_field}"
assumes e: "\<And>e. 0 < e \<Longrightarrow> x \<le> y + e"
shows "x \<le> y"
proof (rule ccontr)
obtain two :: 'a where two: "two = 1 + 1" by simp
assume "\<not> x \<le> y"
then have yx: "y < x" by simp
then have "y + - y < x + - y" by (rule add_strict_right_mono)
then have "x - y > 0" by (simp add: diff_minus)
then have "(x - y) / two > 0"
by (rule divide_pos_pos) (simp add: two)
then have "x \<le> y + (x - y) / two" by (rule e)
also have "... = (x - y + two * y) / two"
also have "... = (x + y) / two"
also have "... < x" using yx
by (simp add: two pos_divide_less_eq algebra_simps)
finally have "x < x" .
then show False ..
qed

code_modulename SML
Fields Arith

code_modulename OCaml
Fields Arith