src/HOL/Fields.thy
 author haftmann Sun, 23 Jun 2013 21:16:07 +0200 changeset 52435 6646bb548c6b parent 44921 58eef4843641 child 53215 5e47c31c6f7c permissions -rw-r--r--
migration from code_(const|type|class|instance) to code_printing and from code_module to code_identifier
```
(*  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

subsection {* Division rings *}

text {*
A division ring is like a field, but without the commutativity requirement.
*}

class inverse =
fixes inverse :: "'a \<Rightarrow> 'a"
and divide :: "'a \<Rightarrow> 'a \<Rightarrow> 'a"  (infixl "'/" 70)

class division_ring = ring_1 + inverse +
assumes left_inverse [simp]:  "a \<noteq> 0 \<Longrightarrow> inverse a * a = 1"
assumes right_inverse [simp]: "a \<noteq> 0 \<Longrightarrow> a * inverse a = 1"
assumes divide_inverse: "a / b = a * inverse b"
begin

subclass 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

lemma nonzero_imp_inverse_nonzero:
"a \<noteq> 0 \<Longrightarrow> inverse a \<noteq> 0"
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" .
thus False by (simp add: eq_commute)
qed

lemma inverse_zero_imp_zero:
"inverse a = 0 \<Longrightarrow> a = 0"
apply (rule classical)
apply (drule nonzero_imp_inverse_nonzero)
apply auto
done

lemma inverse_unique:
assumes ab: "a * b = 1"
shows "inverse a = b"
proof -
have "a \<noteq> 0" using ab by (cases "a = 0") simp_all
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_minus_eq:
"a \<noteq> 0 \<Longrightarrow> inverse (- a) = - inverse a"
by (rule inverse_unique) simp

lemma nonzero_inverse_inverse_eq:
"a \<noteq> 0 \<Longrightarrow> inverse (inverse a) = a"
by (rule inverse_unique) simp

lemma nonzero_inverse_eq_imp_eq:
assumes "inverse a = inverse b" and "a \<noteq> 0" and "b \<noteq> 0"
shows "a = b"
proof -
from `inverse a = inverse b`
have "inverse (inverse a) = inverse (inverse b)" by (rule arg_cong)
with `a \<noteq> 0` and `b \<noteq> 0` show "a = b"
qed

lemma inverse_1 [simp]: "inverse 1 = 1"
by (rule inverse_unique) simp

lemma nonzero_inverse_mult_distrib:
assumes "a \<noteq> 0" and "b \<noteq> 0"
shows "inverse (a * b) = inverse b * inverse a"
proof -
have "a * (b * inverse b) * inverse a = 1" using assms by simp
hence "a * b * (inverse b * inverse a) = 1" by (simp only: mult_assoc)
thus ?thesis by (rule inverse_unique)
qed

"a \<noteq> 0 \<Longrightarrow> b \<noteq> 0 \<Longrightarrow> inverse a + inverse b = inverse a * (a + b) * inverse b"

lemma division_ring_inverse_diff:
"a \<noteq> 0 \<Longrightarrow> b \<noteq> 0 \<Longrightarrow> inverse a - inverse b = inverse a * (b - a) * inverse b"

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_assoc)
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"

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

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

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, no_atp]: "(-a) / b = - (a / b)"

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

lemma nonzero_eq_divide_eq [field_simps]: "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 [field_simps]: "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"

lemma eq_divide_imp: "c \<noteq> 0 \<Longrightarrow> a * c = b \<Longrightarrow> a = b / c"
by (drule sym) (simp add: divide_inverse mult_assoc)

end

class division_ring_inverse_zero = division_ring +
assumes inverse_zero [simp]: "inverse 0 = 0"
begin

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

lemma divide_self_if [simp]:
"a / a = (if a = 0 then 0 else 1)"
by simp

lemma inverse_nonzero_iff_nonzero [simp]:
"inverse a = 0 \<longleftrightarrow> a = 0"
by rule (fact inverse_zero_imp_zero, simp)

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

lemma inverse_inverse_eq [simp]:
"inverse (inverse a) = 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_eq_imp_eq:
"inverse a = inverse b \<Longrightarrow> a = b"
by (drule arg_cong [where f="inverse"], simp)

lemma inverse_eq_iff_eq [simp]:
"inverse a = inverse b \<longleftrightarrow> a = b"
by (force dest!: inverse_eq_imp_eq)

end

subsection {* Fields *}

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 ..

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

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

text{*It's not obvious whether @{text times_divide_eq} 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.*}

lemmas times_divide_eq [no_atp] = 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, no_atp]:
"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, no_atp]:
"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, no_atp]:
"\<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, no_atp]:
"\<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, no_atp]:
"\<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, no_atp]:
"\<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)

"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 [field_simps]:
"z \<noteq> 0 \<Longrightarrow> x - y / z = (z * x - y) / z"

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

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 field_inverse_zero = field +
assumes field_inverse_zero: "inverse 0 = 0"
begin

subclass division_ring_inverse_zero proof
qed (fact field_inverse_zero)

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"
proof cases
assume "a \<noteq> 0 & b \<noteq> 0"
thus ?thesis by (simp add: nonzero_inverse_mult_distrib mult_ac)
next
assume "~ (a \<noteq> 0 & b \<noteq> 0)"
thus ?thesis by force
qed

lemma inverse_divide [simp]:
"inverse (a / b) = b / a"

text {* 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 \<Longrightarrow> (c * a) / (c * b) = a / b"
apply (cases "b = 0")
apply simp_all
done

lemma mult_divide_mult_cancel_right:
"c \<noteq> 0 \<Longrightarrow> (a * c) / (b * c) = a / b"
apply (cases "b = 0")
apply simp_all
done

lemma divide_divide_eq_right [simp, no_atp]:
"a / (b / c) = (a * c) / b"

lemma divide_divide_eq_left [simp, no_atp]:
"(a / b) / c = a / (b * c)"

text {*Special Cancellation Simprules for Division*}

lemma mult_divide_mult_cancel_left_if [simp,no_atp]:
shows "(c * a) / (c * b) = (if c = 0 then 0 else a / b)"

text {* Division and Unary Minus *}

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

lemma divide_minus_right [simp, no_atp]:
"a / - b = - (a / b)"

lemma minus_divide_divide:
"(- a) / (- b) = a / b"
apply (cases "b=0", simp)
done

lemma eq_divide_eq:
"a = b / c \<longleftrightarrow> (if c \<noteq> 0 then a * c = b else a = 0)"

lemma divide_eq_eq:
"b / c = a \<longleftrightarrow> (if c \<noteq> 0 then b = a * c else a = 0)"

lemma inverse_eq_1_iff [simp]:
"inverse x = 1 \<longleftrightarrow> x = 1"
by (insert inverse_eq_iff_eq [of x 1], simp)

lemma divide_eq_0_iff [simp, no_atp]:
"a / b = 0 \<longleftrightarrow> a = 0 \<or> b = 0"

lemma divide_cancel_right [simp, no_atp]:
"a / c = b / c \<longleftrightarrow> c = 0 \<or> a = b"
apply (cases "c=0", simp)
done

lemma divide_cancel_left [simp, no_atp]:
"c / a = c / b \<longleftrightarrow> c = 0 \<or> a = b"
apply (cases "c=0", simp)
done

lemma divide_eq_1_iff [simp, no_atp]:
"a / b = 1 \<longleftrightarrow> b \<noteq> 0 \<and> a = b"
apply (cases "b=0", simp)
done

lemma one_eq_divide_iff [simp, no_atp]:
"1 = a / b \<longleftrightarrow> b \<noteq> 0 \<and> a = b"
by (simp add: eq_commute [of 1])

lemma times_divide_times_eq:
"(x / y) * (z / w) = (x * z) / (y * w)"
by simp

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

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

end

subsection {* Ordered fields *}

class linordered_field = field + linordered_idom
begin

lemma positive_imp_inverse_positive:
assumes a_gt_0: "0 < a"
shows "0 < inverse a"
proof -
have "0 < a * inverse a"
by (simp add: a_gt_0 [THEN less_imp_not_eq2])
thus "0 < inverse a"
by (simp add: a_gt_0 [THEN less_not_sym] zero_less_mult_iff)
qed

lemma negative_imp_inverse_negative:
"a < 0 \<Longrightarrow> inverse a < 0"
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"
proof (rule classical)
assume "~ b \<le> a"
hence "a < b"  by (simp add: linorder_not_le)
hence bpos: "0 < b"  by (blast intro: apos less_trans)
hence "a * inverse a \<le> a * inverse b"
by (simp add: apos invle less_imp_le mult_left_mono)
hence "(a * inverse a) * b \<le> (a * inverse b) * b"
by (simp add: bpos less_imp_le mult_right_mono)
thus "b \<le> a"  by (simp add: mult_assoc apos bpos 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"
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_negative_imp_negative:
assumes inv_less_0: "inverse a < 0" and nz: "a \<noteq> 0"
shows "a < 0"
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 linordered_field_no_lb:
"\<forall>x. \<exists>y. y < x"
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"
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 less_imp_inverse_less:
assumes less: "a < b" and apos:  "0 < a"
shows "inverse b < inverse a"
proof (rule ccontr)
assume "~ inverse b < inverse a"
hence "inverse a \<le> inverse b" by simp
hence "~ (a < b)"
by (simp add: 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 \<Longrightarrow> 0 < a \<Longrightarrow> b < a"
apply (simp add: less_le [of "inverse a"] 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,no_atp]:
"0 < a \<Longrightarrow> 0 < b \<Longrightarrow> inverse a < inverse b \<longleftrightarrow> b < a"
by (blast intro: less_imp_inverse_less dest: inverse_less_imp_less)

lemma le_imp_inverse_le:
"a \<le> b \<Longrightarrow> 0 < a \<Longrightarrow> inverse b \<le> inverse a"
by (force simp add: le_less less_imp_inverse_less)

lemma inverse_le_iff_le [simp,no_atp]:
"0 < a \<Longrightarrow> 0 < b \<Longrightarrow> inverse a \<le> inverse b \<longleftrightarrow> b \<le> a"
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 \<Longrightarrow> b < 0 \<Longrightarrow> b \<le> a"
apply (rule classical)
apply (subgoal_tac "a < 0")
prefer 2 apply force
apply (insert inverse_le_imp_le [of "-b" "-a"])
done

lemma less_imp_inverse_less_neg:
"a < b \<Longrightarrow> b < 0 \<Longrightarrow> inverse b < inverse a"
apply (subgoal_tac "a < 0")
prefer 2 apply (blast intro: less_trans)
apply (insert less_imp_inverse_less [of "-b" "-a"])
done

lemma inverse_less_imp_less_neg:
"inverse a < inverse b \<Longrightarrow> b < 0 \<Longrightarrow> b < a"
apply (rule classical)
apply (subgoal_tac "a < 0")
prefer 2
apply force
apply (insert inverse_less_imp_less [of "-b" "-a"])
done

lemma inverse_less_iff_less_neg [simp,no_atp]:
"a < 0 \<Longrightarrow> b < 0 \<Longrightarrow> inverse a < inverse b \<longleftrightarrow> b < a"
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 \<Longrightarrow> b < 0 ==> inverse b \<le> inverse a"
by (force simp add: le_less less_imp_inverse_less_neg)

lemma inverse_le_iff_le_neg [simp,no_atp]:
"a < 0 \<Longrightarrow> b < 0 \<Longrightarrow> inverse a \<le> inverse b \<longleftrightarrow> b \<le> a"
by (blast intro: le_imp_inverse_le_neg dest: inverse_le_imp_le_neg)

lemma one_less_inverse:
"0 < a \<Longrightarrow> a < 1 \<Longrightarrow> 1 < inverse a"
using less_imp_inverse_less [of a 1, unfolded inverse_1] .

lemma one_le_inverse:
"0 < a \<Longrightarrow> a \<le> 1 \<Longrightarrow> 1 \<le> inverse a"
using le_imp_inverse_le [of a 1, unfolded inverse_1] .

lemma pos_le_divide_eq [field_simps]: "0 < c ==> (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 less_not_sym [OF less] del: times_divide_eq)
also have "... = (a*c \<le> b)"
by (simp add: less_imp_not_eq2 [OF less] divide_inverse mult_assoc)
finally show ?thesis .
qed

lemma neg_le_divide_eq [field_simps]: "c < 0 ==> (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 less_not_sym [OF less] del: times_divide_eq)
also have "... = (b \<le> a*c)"
by (simp add: less_imp_not_eq [OF less] divide_inverse mult_assoc)
finally show ?thesis .
qed

lemma pos_less_divide_eq [field_simps]:
"0 < c ==> (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 less_not_sym [OF less] del: times_divide_eq)
also have "... = (a*c < b)"
by (simp add: less_imp_not_eq2 [OF less] divide_inverse mult_assoc)
finally show ?thesis .
qed

lemma neg_less_divide_eq [field_simps]:
"c < 0 ==> (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 less_not_sym [OF less] del: times_divide_eq)
also have "... = (b < a*c)"
by (simp add: less_imp_not_eq [OF less] divide_inverse mult_assoc)
finally show ?thesis .
qed

lemma pos_divide_less_eq [field_simps]:
"0 < c ==> (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 less_not_sym [OF less] del: times_divide_eq)
also have "... = (b < a*c)"
by (simp add: less_imp_not_eq2 [OF less] divide_inverse mult_assoc)
finally show ?thesis .
qed

lemma neg_divide_less_eq [field_simps]:
"c < 0 ==> (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 less_not_sym [OF less] del: times_divide_eq)
also have "... = (a*c < b)"
by (simp add: less_imp_not_eq [OF less] divide_inverse mult_assoc)
finally show ?thesis .
qed

lemma pos_divide_le_eq [field_simps]: "0 < c ==> (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 less_not_sym [OF less] del: times_divide_eq)
also have "... = (b \<le> a*c)"
by (simp add: less_imp_not_eq2 [OF less] divide_inverse mult_assoc)
finally show ?thesis .
qed

lemma neg_divide_le_eq [field_simps]: "c < 0 ==> (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 less_not_sym [OF less] del: times_divide_eq)
also have "... = (a*c \<le> b)"
by (simp add: less_imp_not_eq [OF less] divide_inverse mult_assoc)
finally show ?thesis .
qed

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 [no_atp] = algebra_simps
zero_less_mult_iff mult_less_0_iff

lemmas (in -) sign_simps [no_atp] = algebra_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
*)

lemma divide_pos_pos:
"0 < x ==> 0 < y ==> 0 < x / y"

lemma divide_nonneg_pos:
"0 <= x ==> 0 < y ==> 0 <= x / y"

lemma divide_neg_pos:
"x < 0 ==> 0 < y ==> x / y < 0"

lemma divide_nonpos_pos:
"x <= 0 ==> 0 < y ==> x / y <= 0"

lemma divide_pos_neg:
"0 < x ==> y < 0 ==> x / y < 0"

lemma divide_nonneg_neg:
"0 <= x ==> y < 0 ==> x / y <= 0"

lemma divide_neg_neg:
"x < 0 ==> y < 0 ==> 0 < x / y"

lemma divide_nonpos_neg:
"x <= 0 ==> y < 0 ==> 0 <= x / y"

lemma divide_strict_right_mono:
"[|a < b; 0 < c|] ==> a / c < b / c"
by (simp add: less_imp_not_eq2 divide_inverse mult_strict_right_mono
positive_imp_inverse_positive)

lemma divide_strict_right_mono_neg:
"[|b < a; c < 0|] ==> a / c < b / c"
apply (drule divide_strict_right_mono [of _ _ "-c"], simp)
apply (simp add: 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"
by (auto simp: field_simps 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"
by (auto simp: field_simps zero_less_mult_iff mult_right_mono)

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

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

lemma mult_imp_le_div_pos: "0 < y ==> z * y <= x ==>
z <= x / y"

lemma mult_imp_div_pos_less: "0 < y ==> x < z * y ==>
x / y < z"

lemma mult_imp_less_div_pos: "0 < y ==> z * y < x ==>
z < x / y"

lemma frac_le: "0 <= 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 <= 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 < x ==>
x <= y ==> 0 < w ==> w < z  ==> x / z < y / w"
apply (rule mult_imp_div_pos_less)
apply simp_all
apply (rule mult_imp_less_div_pos, assumption)
apply (erule mult_le_less_imp_less)
apply simp_all
done

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

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

subclass dense_linorder
proof
fix x y :: 'a
from less_add_one show "\<exists>y. x < y" ..
from less_add_one have "x + (- 1) < (x + 1) + (- 1)" by (rule add_strict_right_mono)
then have "x - 1 < x + 1 - 1" by (simp only: diff_minus [symmetric])
then have "x - 1 < x" by (simp add: algebra_simps)
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

lemma nonzero_abs_inverse:
"a \<noteq> 0 ==> \<bar>inverse a\<bar> = inverse \<bar>a\<bar>"
apply (auto simp add: neq_iff abs_if nonzero_inverse_minus_eq
negative_imp_inverse_negative)
apply (blast intro: positive_imp_inverse_positive elim: less_asym)
done

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

lemma field_le_epsilon:
assumes e: "\<And>e. 0 < e \<Longrightarrow> x \<le> y + e"
shows "x \<le> y"
proof (rule dense_le)
fix t assume "t < x"
hence "0 < x - t" by (simp add: less_diff_eq)
from e [OF this] have "x + 0 \<le> x + (y - t)" by (simp add: algebra_simps)
then have "0 \<le> y - t" by (simp only: add_le_cancel_left)
then show "t \<le> y" by (simp add: algebra_simps)
qed

end

class linordered_field_inverse_zero = linordered_field + field_inverse_zero
begin

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)"
apply (cases "c=0", simp)
apply (force simp add: pos_le_divide_eq neg_le_divide_eq linorder_neq_iff)
done

lemma inverse_positive_iff_positive [simp]:
"(0 < inverse a) = (0 < a)"
apply (cases "a = 0", simp)
apply (blast intro: inverse_positive_imp_positive positive_imp_inverse_positive)
done

lemma inverse_negative_iff_negative [simp]:
"(inverse a < 0) = (a < 0)"
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 \<longleftrightarrow> 0 \<le> a"

lemma inverse_nonpositive_iff_nonpositive [simp]:
"inverse a \<le> 0 \<longleftrightarrow> a \<le> 0"

lemma one_less_inverse_iff:
"1 < inverse x \<longleftrightarrow> 0 < x \<and> x < 1"
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
also have "... < 1" by (rule zero_less_one)
finally show False by auto
qed
with notless show ?thesis by simp
qed

lemma one_le_inverse_iff:
"1 \<le> inverse x \<longleftrightarrow> 0 < x \<and> x \<le> 1"
proof (cases "x = 1")
case True then show ?thesis by simp
next
case False then have "inverse x \<noteq> 1" by simp
then have "1 \<noteq> inverse x" by blast
then have "1 \<le> inverse x \<longleftrightarrow> 1 < inverse x" by (simp add: le_less)
with False show ?thesis by (auto simp add: one_less_inverse_iff)
qed

lemma inverse_less_1_iff:
"inverse x < 1 \<longleftrightarrow> x \<le> 0 \<or> 1 < x"
by (simp add: not_le [symmetric] one_le_inverse_iff)

lemma inverse_le_1_iff:
"inverse x \<le> 1 \<longleftrightarrow> x \<le> 0 \<or> 1 \<le> x"
by (simp add: not_less [symmetric] one_less_inverse_iff)

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)"
apply (cases "c=0", simp)
apply (force simp add: pos_divide_le_eq neg_divide_le_eq)
done

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)"
apply (cases "c=0", simp)
apply (force simp add: pos_less_divide_eq neg_less_divide_eq)
done

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)"
apply (cases "c=0", simp)
apply (force simp add: pos_divide_less_eq neg_divide_less_eq)
done

text {*Division and Signs*}

lemma zero_less_divide_iff:
"(0 < a/b) = (0 < a & 0 < b | a < 0 & b < 0)"

lemma divide_less_0_iff:
"(a/b < 0) =
(0 < a & b < 0 | a < 0 & 0 < b)"

lemma zero_le_divide_iff:
"(0 \<le> a/b) =
(0 \<le> a & 0 \<le> b | a \<le> 0 & b \<le> 0)"

lemma divide_le_0_iff:
"(a/b \<le> 0) =
(0 \<le> a & b \<le> 0 | a \<le> 0 & 0 \<le> b)"

text {* Division and the Number One *}

text{*Simplify expressions equated with 1*}

lemma zero_eq_1_divide_iff [simp,no_atp]:
"(0 = 1/a) = (a = 0)"
apply (cases "a=0", simp)
done

lemma one_divide_eq_0_iff [simp,no_atp]:
"(1/a = 0) = (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"}*}

lemma zero_le_divide_1_iff [simp, no_atp]:
"0 \<le> 1 / a \<longleftrightarrow> 0 \<le> a"

lemma zero_less_divide_1_iff [simp, no_atp]:
"0 < 1 / a \<longleftrightarrow> 0 < a"

lemma divide_le_0_1_iff [simp, no_atp]:
"1 / a \<le> 0 \<longleftrightarrow> a \<le> 0"

lemma divide_less_0_1_iff [simp, no_atp]:
"1 / a < 0 \<longleftrightarrow> a < 0"

lemma divide_right_mono:
"[|a \<le> b; 0 \<le> c|] ==> a/c \<le> b/c"
by (force simp add: divide_strict_right_mono le_less)

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

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

lemma inverse_le_iff:
"inverse a \<le> inverse b \<longleftrightarrow> (0 < a * b \<longrightarrow> b \<le> a) \<and> (a * b \<le> 0 \<longrightarrow> a \<le> b)"
proof -
{ assume "a < 0"
then have "inverse a < 0" by simp
moreover assume "0 < b"
then have "0 < inverse b" by simp
ultimately have "inverse a < inverse b" by (rule less_trans)
then have "inverse a \<le> inverse b" by simp }
moreover
{ assume "b < 0"
then have "inverse b < 0" by simp
moreover assume "0 < a"
then have "0 < inverse a" by simp
ultimately have "inverse b < inverse a" by (rule less_trans)
then have "\<not> inverse a \<le> inverse b" by simp }
ultimately show ?thesis
by (cases a 0 b 0 rule: linorder_cases[case_product linorder_cases])
(auto simp: not_less zero_less_mult_iff mult_le_0_iff)
qed

lemma inverse_less_iff:
"inverse a < inverse b \<longleftrightarrow> (0 < a * b \<longrightarrow> b < a) \<and> (a * b \<le> 0 \<longrightarrow> a < b)"
by (subst less_le) (auto simp: inverse_le_iff)

lemma divide_le_cancel:
"a / c \<le> b / c \<longleftrightarrow> (0 < c \<longrightarrow> a \<le> b) \<and> (c < 0 \<longrightarrow> b \<le> a)"

lemma divide_less_cancel:
"a / c < b / c \<longleftrightarrow> (0 < c \<longrightarrow> a < b) \<and> (c < 0 \<longrightarrow> b < a) \<and> c \<noteq> 0"
by (auto simp add: divide_inverse mult_less_cancel_right)

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

lemma le_divide_eq_1 [no_atp]:
"(1 \<le> b / a) = ((0 < a & a \<le> b) | (a < 0 & b \<le> a))"

lemma divide_le_eq_1 [no_atp]:
"(b / a \<le> 1) = ((0 < a & b \<le> a) | (a < 0 & a \<le> b) | a=0)"

lemma less_divide_eq_1 [no_atp]:
"(1 < b / a) = ((0 < a & a < b) | (a < 0 & b < a))"

lemma divide_less_eq_1 [no_atp]:
"(b / a < 1) = ((0 < a & b < a) | (a < 0 & a < b) | a=0)"

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

lemma le_divide_eq_1_pos [simp,no_atp]:
"0 < a \<Longrightarrow> (1 \<le> b/a) = (a \<le> b)"

lemma le_divide_eq_1_neg [simp,no_atp]:
"a < 0 \<Longrightarrow> (1 \<le> b/a) = (b \<le> a)"

lemma divide_le_eq_1_pos [simp,no_atp]:
"0 < a \<Longrightarrow> (b/a \<le> 1) = (b \<le> a)"

lemma divide_le_eq_1_neg [simp,no_atp]:
"a < 0 \<Longrightarrow> (b/a \<le> 1) = (a \<le> b)"

lemma less_divide_eq_1_pos [simp,no_atp]:
"0 < a \<Longrightarrow> (1 < b/a) = (a < b)"

lemma less_divide_eq_1_neg [simp,no_atp]:
"a < 0 \<Longrightarrow> (1 < b/a) = (b < a)"

lemma divide_less_eq_1_pos [simp,no_atp]:
"0 < a \<Longrightarrow> (b/a < 1) = (b < a)"

lemma divide_less_eq_1_neg [simp,no_atp]:
"a < 0 \<Longrightarrow> b/a < 1 <-> a < b"

lemma eq_divide_eq_1 [simp,no_atp]:
"(1 = b/a) = ((a \<noteq> 0 & a = b))"

lemma divide_eq_eq_1 [simp,no_atp]:
"(b/a = 1) = ((a \<noteq> 0 & a = b))"

lemma abs_inverse [simp]:
"\<bar>inverse a\<bar> =
inverse \<bar>a\<bar>"
apply (cases "a=0", simp)
done

lemma abs_divide [simp]:
"\<bar>a / b\<bar> = \<bar>a\<bar> / \<bar>b\<bar>"
apply (cases "b=0", simp)
done

lemma abs_div_pos: "0 < y ==>
\<bar>x\<bar> / y = \<bar>x / y\<bar>"
apply (subst abs_divide)
done

lemma field_le_mult_one_interval:
assumes *: "\<And>z. \<lbrakk> 0 < z ; z < 1 \<rbrakk> \<Longrightarrow> z * x \<le> y"
shows "x \<le> y"
proof (cases "0 < x")
assume "0 < x"
thus ?thesis
using dense_le_bounded[of 0 1 "y/x"] *
unfolding le_divide_eq if_P[OF `0 < x`] by simp
next
assume "\<not>0 < x" hence "x \<le> 0" by simp
obtain s::'a where s: "0 < s" "s < 1" using dense[of 0 "1\<Colon>'a"] by auto
hence "x \<le> s * x" using mult_le_cancel_right[of 1 x s] `x \<le> 0` by auto
also note *[OF s]
finally show ?thesis .
qed

end

code_identifier
code_module Fields \<rightharpoonup> (SML) Arith and (OCaml) Arith and (Haskell) Arith

end
```