(* Title: HOL/ex/Puzzle.thy
ID: $Id$
Author: Tobias Nipkow
Copyright 1993 TU Muenchen
A question from "Bundeswettbewerb Mathematik"
Proof due to Herbert Ehler
*)
theory Puzzle = Main:
consts f :: "nat => nat"
axioms f_ax [intro!]: "f(f(n)) < f(Suc(n))"
lemma lemma0 [rule_format]: "\<forall>n. k=f(n) --> n <= f(n)"
apply (induct_tac "k" rule: nat_less_induct)
apply (rule allI)
apply (rename_tac "i")
apply (case_tac "i")
apply simp
apply (blast intro!: Suc_leI intro: le_less_trans)
done
lemma lemma1: "n <= f(n)"
by (blast intro: lemma0)
lemma lemma2: "f(n) < f(Suc(n))"
by (blast intro: le_less_trans lemma1)
lemma f_mono [rule_format (no_asm)]: "m <= n --> f(m) <= f(n)"
apply (induct_tac "n")
apply simp
apply (rule impI)
apply (erule le_SucE)
apply (cut_tac n = n in lemma2, auto)
done
lemma f_id: "f(n) = n"
apply (rule order_antisym)
apply (rule_tac [2] lemma1)
apply (blast intro: leI dest: leD f_mono Suc_leI)
done
end