--- a/src/HOL/ex/Sqrt.thy Sat Jun 05 12:29:57 2021 +0200
+++ b/src/HOL/ex/Sqrt.thy Sat Jun 05 12:45:00 2021 +0200
@@ -1,42 +1,46 @@
(* Title: HOL/ex/Sqrt.thy
- Author: Markus Wenzel, Tobias Nipkow, TU Muenchen
+ Author: Makarius
+ Author: Tobias Nipkow, TU Muenchen
*)
section \<open>Square roots of primes are irrational\<close>
theory Sqrt
-imports Complex_Main "HOL-Computational_Algebra.Primes"
+ imports Complex_Main "HOL-Computational_Algebra.Primes"
begin
-text \<open>The square root of any prime number (including 2) is irrational.\<close>
+text \<open>
+ The square root of any prime number (including 2) is irrational.
+\<close>
theorem sqrt_prime_irrational:
- assumes "prime (p::nat)"
+ fixes p :: nat
+ assumes "prime p"
shows "sqrt p \<notin> \<rat>"
proof
- from \<open>prime p\<close> have p: "1 < p" by (simp add: prime_nat_iff)
+ from \<open>prime p\<close> have p: "p > 1" by (rule prime_gt_1_nat)
assume "sqrt p \<in> \<rat>"
- then obtain m n :: nat where
- n: "n \<noteq> 0" and sqrt_rat: "\<bar>sqrt p\<bar> = m / n"
- and "coprime m n" by (rule Rats_abs_nat_div_natE)
+ then obtain m n :: nat
+ where n: "n \<noteq> 0"
+ and sqrt_rat: "\<bar>sqrt p\<bar> = m / n"
+ and "coprime m n" by (rule Rats_abs_nat_div_natE)
have eq: "m\<^sup>2 = p * n\<^sup>2"
proof -
from n and sqrt_rat have "m = \<bar>sqrt p\<bar> * n" by simp
- then have "m\<^sup>2 = (sqrt p)\<^sup>2 * n\<^sup>2"
- by (auto simp add: power2_eq_square)
+ then have "m\<^sup>2 = (sqrt p)\<^sup>2 * n\<^sup>2" by (simp add: power_mult_distrib)
also have "(sqrt p)\<^sup>2 = p" by simp
also have "\<dots> * n\<^sup>2 = p * n\<^sup>2" by simp
- finally show ?thesis using of_nat_eq_iff by blast
+ finally show ?thesis by linarith
qed
have "p dvd m \<and> p dvd n"
proof
from eq have "p dvd m\<^sup>2" ..
- with \<open>prime p\<close> show "p dvd m" by (rule prime_dvd_power_nat)
+ with \<open>prime p\<close> show "p dvd m" by (rule prime_dvd_power)
then obtain k where "m = p * k" ..
- with eq have "p * n\<^sup>2 = p\<^sup>2 * k\<^sup>2" by (auto simp add: power2_eq_square ac_simps)
+ with eq have "p * n\<^sup>2 = p\<^sup>2 * k\<^sup>2" by algebra
with p have "n\<^sup>2 = p * k\<^sup>2" by (simp add: power2_eq_square)
then have "p dvd n\<^sup>2" ..
- with \<open>prime p\<close> show "p dvd n" by (rule prime_dvd_power_nat)
+ with \<open>prime p\<close> show "p dvd n" by (rule prime_dvd_power)
qed
then have "p dvd gcd m n" by simp
with \<open>coprime m n\<close> have "p = 1" by simp
@@ -44,46 +48,47 @@
qed
corollary sqrt_2_not_rat: "sqrt 2 \<notin> \<rat>"
- using sqrt_prime_irrational[of 2] by simp
-
-
-subsection \<open>Variations\<close>
+ using sqrt_prime_irrational [of 2] by simp
text \<open>
- Here is an alternative version of the main proof, using mostly
- linear forward-reasoning. While this results in less top-down
- structure, it is probably closer to proofs seen in mathematics.
+ Here is an alternative version of the main proof, using mostly linear
+ forward-reasoning. While this results in less top-down structure, it is
+ probably closer to proofs seen in mathematics.
\<close>
theorem
- assumes "prime (p::nat)"
+ fixes p :: nat
+ assumes "prime p"
shows "sqrt p \<notin> \<rat>"
proof
- from \<open>prime p\<close> have p: "1 < p" by (simp add: prime_nat_iff)
+ from \<open>prime p\<close> have p: "p > 1" by (rule prime_gt_1_nat)
assume "sqrt p \<in> \<rat>"
- then obtain m n :: nat where
- n: "n \<noteq> 0" and sqrt_rat: "\<bar>sqrt p\<bar> = m / n"
- and "coprime m n" by (rule Rats_abs_nat_div_natE)
+ then obtain m n :: nat
+ where n: "n \<noteq> 0"
+ and sqrt_rat: "\<bar>sqrt p\<bar> = m / n"
+ and "coprime m n" by (rule Rats_abs_nat_div_natE)
from n and sqrt_rat have "m = \<bar>sqrt p\<bar> * n" by simp
- then have "m\<^sup>2 = (sqrt p)\<^sup>2 * n\<^sup>2"
- by (auto simp add: power2_eq_square)
+ then have "m\<^sup>2 = (sqrt p)\<^sup>2 * n\<^sup>2" by (auto simp add: power2_eq_square)
also have "(sqrt p)\<^sup>2 = p" by simp
also have "\<dots> * n\<^sup>2 = p * n\<^sup>2" by simp
- finally have eq: "m\<^sup>2 = p * n\<^sup>2" using of_nat_eq_iff by blast
+ finally have eq: "m\<^sup>2 = p * n\<^sup>2" by linarith
then have "p dvd m\<^sup>2" ..
- with \<open>prime p\<close> have dvd_m: "p dvd m" by (rule prime_dvd_power_nat)
+ with \<open>prime p\<close> have dvd_m: "p dvd m" by (rule prime_dvd_power)
then obtain k where "m = p * k" ..
- with eq have "p * n\<^sup>2 = p\<^sup>2 * k\<^sup>2" by (auto simp add: power2_eq_square ac_simps)
+ with eq have "p * n\<^sup>2 = p\<^sup>2 * k\<^sup>2" by algebra
with p have "n\<^sup>2 = p * k\<^sup>2" by (simp add: power2_eq_square)
then have "p dvd n\<^sup>2" ..
- with \<open>prime p\<close> have "p dvd n" by (rule prime_dvd_power_nat)
+ with \<open>prime p\<close> have "p dvd n" by (rule prime_dvd_power)
with dvd_m have "p dvd gcd m n" by (rule gcd_greatest)
with \<open>coprime m n\<close> have "p = 1" by simp
with p show False by simp
qed
-text \<open>Another old chestnut, which is a consequence of the irrationality of \<^term>\<open>sqrt 2\<close>.\<close>
+text \<open>
+ Another old chestnut, which is a consequence of the irrationality of
+ \<^term>\<open>sqrt 2\<close>.
+\<close>
lemma "\<exists>a b::real. a \<notin> \<rat> \<and> b \<notin> \<rat> \<and> a powr b \<in> \<rat>" (is "\<exists>a b. ?P a b")
proof (cases "sqrt 2 powr sqrt 2 \<in> \<rat>")