
paulson@3390: (*  Title:      HOL/Power.thy
paulson@3390:     Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
paulson@3390:     Copyright   1997  University of Cambridge
paulson@3390: *)
paulson@3390: 
haftmann@30960: header {* Exponentiation *}
paulson@14348: 
nipkow@15131: theory Power
huffman@47191: imports Num
nipkow@15131: begin
paulson@14348: 
haftmann@30960: subsection {* Powers for Arbitrary Monoids *}
haftmann@30960: 
haftmann@30996: class power = one + times
haftmann@30960: begin
haftmann@24996: 
haftmann@30960: primrec power :: "'a \<Rightarrow> nat \<Rightarrow> 'a" (infixr "^" 80) where
haftmann@30960:     power_0: "a ^ 0 = 1"
haftmann@30960:   | power_Suc: "a ^ Suc n = a * a ^ n"
paulson@14348: 
haftmann@30996: notation (latex output)
haftmann@30996:   power ("(_\<^bsup>_\<^esup>)" [1000] 1000)
haftmann@30996: 
haftmann@30996: notation (HTML output)
haftmann@30996:   power ("(_\<^bsup>_\<^esup>)" [1000] 1000)
haftmann@30996: 
huffman@47192: text {* Special syntax for squares. *}
huffman@47192: 
huffman@47192: abbreviation (xsymbols)
wenzelm@53015:   power2 :: "'a \<Rightarrow> 'a"  ("(_\<^sup>2)" [1000] 999) where
wenzelm@53015:   "x\<^sup>2 \<equiv> x ^ 2"
huffman@47192: 
huffman@47192: notation (latex output)
wenzelm@53015:   power2  ("(_\<^sup>2)" [1000] 999)
huffman@47192: 
huffman@47192: notation (HTML output)
wenzelm@53015:   power2  ("(_\<^sup>2)" [1000] 999)
huffman@47192: 
haftmann@30960: end
paulson@14348: 
haftmann@30996: context monoid_mult
haftmann@30996: begin
paulson@14348: 
wenzelm@39438: subclass power .
paulson@14348: 
haftmann@30996: lemma power_one [simp]:
haftmann@30996:   "1 ^ n = 1"
huffman@30273:   by (induct n) simp_all
paulson@14348: 
haftmann@30996: lemma power_one_right [simp]:
haftmann@31001:   "a ^ 1 = a"
haftmann@30996:   by simp
paulson@14348: 
haftmann@30996: lemma power_commutes:
haftmann@30996:   "a ^ n * a = a * a ^ n"
huffman@30273:   by (induct n) (simp_all add: mult_assoc)
krauss@21199: 
haftmann@30996: lemma power_Suc2:
haftmann@30996:   "a ^ Suc n = a ^ n * a"
huffman@30273:   by (simp add: power_commutes)
huffman@28131: 
haftmann@30996: lemma power_add:
haftmann@30996:   "a ^ (m + n) = a ^ m * a ^ n"
haftmann@30996:   by (induct m) (simp_all add: algebra_simps)
paulson@14348: 
haftmann@30996: lemma power_mult:
haftmann@30996:   "a ^ (m * n) = (a ^ m) ^ n"
huffman@30273:   by (induct n) (simp_all add: power_add)
paulson@14348: 
wenzelm@53015: lemma power2_eq_square: "a\<^sup>2 = a * a"
huffman@47192:   by (simp add: numeral_2_eq_2)
huffman@47192: 
huffman@47192: lemma power3_eq_cube: "a ^ 3 = a * a * a"
huffman@47192:   by (simp add: numeral_3_eq_3 mult_assoc)
huffman@47192: 
huffman@47192: lemma power_even_eq:
wenzelm@53076:   "a ^ (2 * n) = (a ^ n)\<^sup>2"
huffman@47192:   by (subst mult_commute) (simp add: power_mult)
huffman@47192: 
huffman@47192: lemma power_odd_eq:
wenzelm@53076:   "a ^ Suc (2*n) = a * (a ^ n)\<^sup>2"
huffman@47192:   by (simp add: power_even_eq)
huffman@47192: 
huffman@47255: lemma power_numeral_even:
huffman@47255:   "z ^ numeral (Num.Bit0 w) = (let w = z ^ (numeral w) in w * w)"
huffman@47255:   unfolding numeral_Bit0 power_add Let_def ..
huffman@47255: 
huffman@47255: lemma power_numeral_odd:
huffman@47255:   "z ^ numeral (Num.Bit1 w) = (let w = z ^ (numeral w) in z * w * w)"
huffman@47255:   unfolding numeral_Bit1 One_nat_def add_Suc_right add_0_right
huffman@47255:   unfolding power_Suc power_add Let_def mult_assoc ..
huffman@47255: 
haftmann@49824: lemma funpow_times_power:
haftmann@49824:   "(times x ^^ f x) = times (x ^ f x)"
haftmann@49824: proof (induct "f x" arbitrary: f)
haftmann@49824:   case 0 then show ?case by (simp add: fun_eq_iff)
haftmann@49824: next
haftmann@49824:   case (Suc n)
haftmann@49824:   def g \<equiv> "\<lambda>x. f x - 1"
haftmann@49824:   with Suc have "n = g x" by simp
haftmann@49824:   with Suc have "times x ^^ g x = times (x ^ g x)" by simp
haftmann@49824:   moreover from Suc g_def have "f x = g x + 1" by simp
haftmann@49824:   ultimately show ?case by (simp add: power_add funpow_add fun_eq_iff mult_assoc)
haftmann@49824: qed
haftmann@49824: 
haftmann@30996: end
haftmann@30996: 
haftmann@30996: context comm_monoid_mult
haftmann@30996: begin
haftmann@30996: 
haftmann@30996: lemma power_mult_distrib:
haftmann@30996:   "(a * b) ^ n = (a ^ n) * (b ^ n)"
huffman@30273:   by (induct n) (simp_all add: mult_ac)
paulson@14348: 
haftmann@30996: end
haftmann@30996: 
huffman@47191: context semiring_numeral
huffman@47191: begin
huffman@47191: 
huffman@47191: lemma numeral_sqr: "numeral (Num.sqr k) = numeral k * numeral k"
huffman@47191:   by (simp only: sqr_conv_mult numeral_mult)
huffman@47191: 
huffman@47191: lemma numeral_pow: "numeral (Num.pow k l) = numeral k ^ numeral l"
huffman@47191:   by (induct l, simp_all only: numeral_class.numeral.simps pow.simps
huffman@47191:     numeral_sqr numeral_mult power_add power_one_right)
huffman@47191: 
huffman@47191: lemma power_numeral [simp]: "numeral k ^ numeral l = numeral (Num.pow k l)"
huffman@47191:   by (rule numeral_pow [symmetric])
huffman@47191: 
huffman@47191: end
huffman@47191: 
haftmann@30996: context semiring_1
haftmann@30996: begin
haftmann@30996: 
haftmann@30996: lemma of_nat_power:
haftmann@30996:   "of_nat (m ^ n) = of_nat m ^ n"
haftmann@30996:   by (induct n) (simp_all add: of_nat_mult)
haftmann@30996: 
huffman@47191: lemma power_zero_numeral [simp]: "(0::'a) ^ numeral k = 0"
huffman@47209:   by (simp add: numeral_eq_Suc)
huffman@47191: 
wenzelm@53015: lemma zero_power2: "0\<^sup>2 = 0" (* delete? *)
huffman@47192:   by (rule power_zero_numeral)
huffman@47192: 
wenzelm@53015: lemma one_power2: "1\<^sup>2 = 1" (* delete? *)
huffman@47192:   by (rule power_one)
huffman@47192: 
haftmann@30996: end
haftmann@30996: 
haftmann@30996: context comm_semiring_1
haftmann@30996: begin
haftmann@30996: 
haftmann@30996: text {* The divides relation *}
haftmann@30996: 
haftmann@30996: lemma le_imp_power_dvd:
haftmann@30996:   assumes "m \<le> n" shows "a ^ m dvd a ^ n"
haftmann@30996: proof
haftmann@30996:   have "a ^ n = a ^ (m + (n - m))"
haftmann@30996:     using `m \<le> n` by simp
haftmann@30996:   also have "\<dots> = a ^ m * a ^ (n - m)"
haftmann@30996:     by (rule power_add)
haftmann@30996:   finally show "a ^ n = a ^ m * a ^ (n - m)" .
haftmann@30996: qed
haftmann@30996: 
haftmann@30996: lemma power_le_dvd:
haftmann@30996:   "a ^ n dvd b \<Longrightarrow> m \<le> n \<Longrightarrow> a ^ m dvd b"
haftmann@30996:   by (rule dvd_trans [OF le_imp_power_dvd])
haftmann@30996: 
haftmann@30996: lemma dvd_power_same:
haftmann@30996:   "x dvd y \<Longrightarrow> x ^ n dvd y ^ n"
haftmann@30996:   by (induct n) (auto simp add: mult_dvd_mono)
haftmann@30996: 
haftmann@30996: lemma dvd_power_le:
haftmann@30996:   "x dvd y \<Longrightarrow> m \<ge> n \<Longrightarrow> x ^ n dvd y ^ m"
haftmann@30996:   by (rule power_le_dvd [OF dvd_power_same])
paulson@14348: 
haftmann@30996: lemma dvd_power [simp]:
haftmann@30996:   assumes "n > (0::nat) \<or> x = 1"
haftmann@30996:   shows "x dvd (x ^ n)"
haftmann@30996: using assms proof
haftmann@30996:   assume "0 < n"
haftmann@30996:   then have "x ^ n = x ^ Suc (n - 1)" by simp
haftmann@30996:   then show "x dvd (x ^ n)" by simp
haftmann@30996: next
haftmann@30996:   assume "x = 1"
haftmann@30996:   then show "x dvd (x ^ n)" by simp
haftmann@30996: qed
haftmann@30996: 
haftmann@30996: end
haftmann@30996: 
haftmann@30996: context ring_1
haftmann@30996: begin
haftmann@30996: 
haftmann@30996: lemma power_minus:
haftmann@30996:   "(- a) ^ n = (- 1) ^ n * a ^ n"
haftmann@30996: proof (induct n)
haftmann@30996:   case 0 show ?case by simp
haftmann@30996: next
haftmann@30996:   case (Suc n) then show ?case
haftmann@30996:     by (simp del: power_Suc add: power_Suc2 mult_assoc)
haftmann@30996: qed
haftmann@30996: 
huffman@47191: lemma power_minus_Bit0:
huffman@47191:   "(- x) ^ numeral (Num.Bit0 k) = x ^ numeral (Num.Bit0 k)"
huffman@47191:   by (induct k, simp_all only: numeral_class.numeral.simps power_add
huffman@47191:     power_one_right mult_minus_left mult_minus_right minus_minus)
huffman@47191: 
huffman@47191: lemma power_minus_Bit1:
huffman@47191:   "(- x) ^ numeral (Num.Bit1 k) = - (x ^ numeral (Num.Bit1 k))"
huffman@47220:   by (simp only: eval_nat_numeral(3) power_Suc power_minus_Bit0 mult_minus_left)
huffman@47191: 
huffman@47191: lemma power_neg_numeral_Bit0 [simp]:
huffman@47191:   "neg_numeral k ^ numeral (Num.Bit0 l) = numeral (Num.pow k (Num.Bit0 l))"
huffman@47191:   by (simp only: neg_numeral_def power_minus_Bit0 power_numeral)
huffman@47191: 
huffman@47191: lemma power_neg_numeral_Bit1 [simp]:
huffman@47191:   "neg_numeral k ^ numeral (Num.Bit1 l) = neg_numeral (Num.pow k (Num.Bit1 l))"
huffman@47191:   by (simp only: neg_numeral_def power_minus_Bit1 power_numeral pow.simps)
huffman@47191: 
huffman@47192: lemma power2_minus [simp]:
wenzelm@53015:   "(- a)\<^sup>2 = a\<^sup>2"
huffman@47192:   by (rule power_minus_Bit0)
huffman@47192: 
huffman@47192: lemma power_minus1_even [simp]:
huffman@47192:   "-1 ^ (2*n) = 1"
huffman@47192: proof (induct n)
huffman@47192:   case 0 show ?case by simp
huffman@47192: next
huffman@47192:   case (Suc n) then show ?case by (simp add: power_add power2_eq_square)
huffman@47192: qed
huffman@47192: 
huffman@47192: lemma power_minus1_odd:
huffman@47192:   "-1 ^ Suc (2*n) = -1"
huffman@47192:   by simp
huffman@47192: 
huffman@47192: lemma power_minus_even [simp]:
huffman@47192:   "(-a) ^ (2*n) = a ^ (2*n)"
huffman@47192:   by (simp add: power_minus [of a])
huffman@47192: 
huffman@47192: end
huffman@47192: 
huffman@47192: context ring_1_no_zero_divisors
huffman@47192: begin
huffman@47192: 
huffman@47192: lemma field_power_not_zero:
huffman@47192:   "a \<noteq> 0 \<Longrightarrow> a ^ n \<noteq> 0"
huffman@47192:   by (induct n) auto
huffman@47192: 
huffman@47192: lemma zero_eq_power2 [simp]:
wenzelm@53015:   "a\<^sup>2 = 0 \<longleftrightarrow> a = 0"
huffman@47192:   unfolding power2_eq_square by simp
huffman@47192: 
huffman@47192: lemma power2_eq_1_iff:
wenzelm@53015:   "a\<^sup>2 = 1 \<longleftrightarrow> a = 1 \<or> a = - 1"
huffman@47192:   unfolding power2_eq_square by (rule square_eq_1_iff)
huffman@47192: 
huffman@47192: end
huffman@47192: 
huffman@47192: context idom
huffman@47192: begin
huffman@47192: 
wenzelm@53015: lemma power2_eq_iff: "x\<^sup>2 = y\<^sup>2 \<longleftrightarrow> x = y \<or> x = - y"
huffman@47192:   unfolding power2_eq_square by (rule square_eq_iff)
huffman@47192: 
huffman@47192: end
huffman@47192: 
huffman@47192: context division_ring
huffman@47192: begin
huffman@47192: 
huffman@47192: text {* FIXME reorient or rename to @{text nonzero_inverse_power} *}
huffman@47192: lemma nonzero_power_inverse:
huffman@47192:   "a \<noteq> 0 \<Longrightarrow> inverse (a ^ n) = (inverse a) ^ n"
huffman@47192:   by (induct n)
huffman@47192:     (simp_all add: nonzero_inverse_mult_distrib power_commutes field_power_not_zero)
huffman@47192: 
huffman@47192: end
huffman@47192: 
huffman@47192: context field
huffman@47192: begin
huffman@47192: 
huffman@47192: lemma nonzero_power_divide:
huffman@47192:   "b \<noteq> 0 \<Longrightarrow> (a / b) ^ n = a ^ n / b ^ n"
huffman@47192:   by (simp add: divide_inverse power_mult_distrib nonzero_power_inverse)
huffman@47192: 
huffman@47192: end
huffman@47192: 
huffman@47192: 
huffman@47192: subsection {* Exponentiation on ordered types *}
huffman@47192: 
huffman@47192: context linordered_ring (* TODO: move *)
huffman@47192: begin
huffman@47192: 
huffman@47192: lemma sum_squares_ge_zero:
huffman@47192:   "0 \<le> x * x + y * y"
huffman@47192:   by (intro add_nonneg_nonneg zero_le_square)
huffman@47192: 
huffman@47192: lemma not_sum_squares_lt_zero:
huffman@47192:   "\<not> x * x + y * y < 0"
huffman@47192:   by (simp add: not_less sum_squares_ge_zero)
huffman@47192: 
haftmann@30996: end
haftmann@30996: 
haftmann@35028: context linordered_semidom
haftmann@30996: begin
haftmann@30996: 
haftmann@30996: lemma zero_less_power [simp]:
haftmann@30996:   "0 < a \<Longrightarrow> 0 < a ^ n"
haftmann@30996:   by (induct n) (simp_all add: mult_pos_pos)
haftmann@30996: 
haftmann@30996: lemma zero_le_power [simp]:
haftmann@30996:   "0 \<le> a \<Longrightarrow> 0 \<le> a ^ n"
haftmann@30996:   by (induct n) (simp_all add: mult_nonneg_nonneg)
paulson@14348: 
huffman@47241: lemma power_mono:
huffman@47241:   "a \<le> b \<Longrightarrow> 0 \<le> a \<Longrightarrow> a ^ n \<le> b ^ n"
huffman@47241:   by (induct n) (auto intro: mult_mono order_trans [of 0 a b])
huffman@47241: 
huffman@47241: lemma one_le_power [simp]: "1 \<le> a \<Longrightarrow> 1 \<le> a ^ n"
huffman@47241:   using power_mono [of 1 a n] by simp
huffman@47241: 
huffman@47241: lemma power_le_one: "\<lbrakk>0 \<le> a; a \<le> 1\<rbrakk> \<Longrightarrow> a ^ n \<le> 1"
huffman@47241:   using power_mono [of a 1 n] by simp
paulson@14348: 
paulson@14348: lemma power_gt1_lemma:
haftmann@30996:   assumes gt1: "1 < a"
haftmann@30996:   shows "1 < a * a ^ n"
paulson@14348: proof -
haftmann@30996:   from gt1 have "0 \<le> a"
haftmann@30996:     by (fact order_trans [OF zero_le_one less_imp_le])
haftmann@30996:   have "1 * 1 < a * 1" using gt1 by simp
haftmann@30996:   also have "\<dots> \<le> a * a ^ n" using gt1
haftmann@30996:     by (simp only: mult_mono `0 \<le> a` one_le_power order_less_imp_le
wenzelm@14577:         zero_le_one order_refl)
wenzelm@14577:   finally show ?thesis by simp
paulson@14348: qed
paulson@14348: 
haftmann@30996: lemma power_gt1:
haftmann@30996:   "1 < a \<Longrightarrow> 1 < a ^ Suc n"
haftmann@30996:   by (simp add: power_gt1_lemma)
huffman@24376: 
haftmann@30996: lemma one_less_power [simp]:
haftmann@30996:   "1 < a \<Longrightarrow> 0 < n \<Longrightarrow> 1 < a ^ n"
haftmann@30996:   by (cases n) (simp_all add: power_gt1_lemma)
paulson@14348: 
paulson@14348: lemma power_le_imp_le_exp:
haftmann@30996:   assumes gt1: "1 < a"
haftmann@30996:   shows "a ^ m \<le> a ^ n \<Longrightarrow> m \<le> n"
haftmann@30996: proof (induct m arbitrary: n)
paulson@14348:   case 0
wenzelm@14577:   show ?case by simp
paulson@14348: next
paulson@14348:   case (Suc m)
wenzelm@14577:   show ?case
wenzelm@14577:   proof (cases n)
wenzelm@14577:     case 0
haftmann@30996:     with Suc.prems Suc.hyps have "a * a ^ m \<le> 1" by simp
wenzelm@14577:     with gt1 show ?thesis
wenzelm@14577:       by (force simp only: power_gt1_lemma
haftmann@30996:           not_less [symmetric])
wenzelm@14577:   next
wenzelm@14577:     case (Suc n)
haftmann@30996:     with Suc.prems Suc.hyps show ?thesis
wenzelm@14577:       by (force dest: mult_left_le_imp_le
haftmann@30996:           simp add: less_trans [OF zero_less_one gt1])
wenzelm@14577:   qed
paulson@14348: qed
paulson@14348: 
wenzelm@14577: text{*Surely we can strengthen this? It holds for @{text "0<a<1"} too.*}
paulson@14348: lemma power_inject_exp [simp]:
haftmann@30996:   "1 < a \<Longrightarrow> a ^ m = a ^ n \<longleftrightarrow> m = n"
wenzelm@14577:   by (force simp add: order_antisym power_le_imp_le_exp)
paulson@14348: 
paulson@14348: text{*Can relax the first premise to @{term "0<a"} in the case of the
paulson@14348: natural numbers.*}
paulson@14348: lemma power_less_imp_less_exp:
haftmann@30996:   "1 < a \<Longrightarrow> a ^ m < a ^ n \<Longrightarrow> m < n"
haftmann@30996:   by (simp add: order_less_le [of m n] less_le [of "a^m" "a^n"]
haftmann@30996:     power_le_imp_le_exp)
paulson@14348: 
paulson@14348: lemma power_strict_mono [rule_format]:
haftmann@30996:   "a < b \<Longrightarrow> 0 \<le> a \<Longrightarrow> 0 < n \<longrightarrow> a ^ n < b ^ n"
haftmann@30996:   by (induct n)
haftmann@30996:    (auto simp add: mult_strict_mono le_less_trans [of 0 a b])
paulson@14348: 
paulson@14348: text{*Lemma for @{text power_strict_decreasing}*}
paulson@14348: lemma power_Suc_less:
haftmann@30996:   "0 < a \<Longrightarrow> a < 1 \<Longrightarrow> a * a ^ n < a ^ n"
haftmann@30996:   by (induct n)
haftmann@30996:     (auto simp add: mult_strict_left_mono)
paulson@14348: 
haftmann@30996: lemma power_strict_decreasing [rule_format]:
haftmann@30996:   "n < N \<Longrightarrow> 0 < a \<Longrightarrow> a < 1 \<longrightarrow> a ^ N < a ^ n"
haftmann@30996: proof (induct N)
haftmann@30996:   case 0 then show ?case by simp
haftmann@30996: next
haftmann@30996:   case (Suc N) then show ?case 
haftmann@30996:   apply (auto simp add: power_Suc_less less_Suc_eq)
haftmann@30996:   apply (subgoal_tac "a * a^N < 1 * a^n")
haftmann@30996:   apply simp
haftmann@30996:   apply (rule mult_strict_mono) apply auto
haftmann@30996:   done
haftmann@30996: qed
paulson@14348: 
paulson@14348: text{*Proof resembles that of @{text power_strict_decreasing}*}
haftmann@30996: lemma power_decreasing [rule_format]:
haftmann@30996:   "n \<le> N \<Longrightarrow> 0 \<le> a \<Longrightarrow> a \<le> 1 \<longrightarrow> a ^ N \<le> a ^ n"
haftmann@30996: proof (induct N)
haftmann@30996:   case 0 then show ?case by simp
haftmann@30996: next
haftmann@30996:   case (Suc N) then show ?case 
haftmann@30996:   apply (auto simp add: le_Suc_eq)
haftmann@30996:   apply (subgoal_tac "a * a^N \<le> 1 * a^n", simp)
haftmann@30996:   apply (rule mult_mono) apply auto
haftmann@30996:   done
haftmann@30996: qed
paulson@14348: 
paulson@14348: lemma power_Suc_less_one:
haftmann@30996:   "0 < a \<Longrightarrow> a < 1 \<Longrightarrow> a ^ Suc n < 1"
haftmann@30996:   using power_strict_decreasing [of 0 "Suc n" a] by simp
paulson@14348: 
paulson@14348: text{*Proof again resembles that of @{text power_strict_decreasing}*}
haftmann@30996: lemma power_increasing [rule_format]:
haftmann@30996:   "n \<le> N \<Longrightarrow> 1 \<le> a \<Longrightarrow> a ^ n \<le> a ^ N"
haftmann@30996: proof (induct N)
haftmann@30996:   case 0 then show ?case by simp
haftmann@30996: next
haftmann@30996:   case (Suc N) then show ?case 
haftmann@30996:   apply (auto simp add: le_Suc_eq)
haftmann@30996:   apply (subgoal_tac "1 * a^n \<le> a * a^N", simp)
haftmann@30996:   apply (rule mult_mono) apply (auto simp add: order_trans [OF zero_le_one])
haftmann@30996:   done
haftmann@30996: qed
paulson@14348: 
paulson@14348: text{*Lemma for @{text power_strict_increasing}*}
paulson@14348: lemma power_less_power_Suc:
haftmann@30996:   "1 < a \<Longrightarrow> a ^ n < a * a ^ n"
haftmann@30996:   by (induct n) (auto simp add: mult_strict_left_mono less_trans [OF zero_less_one])
paulson@14348: 
haftmann@30996: lemma power_strict_increasing [rule_format]:
haftmann@30996:   "n < N \<Longrightarrow> 1 < a \<longrightarrow> a ^ n < a ^ N"
haftmann@30996: proof (induct N)
haftmann@30996:   case 0 then show ?case by simp
haftmann@30996: next
haftmann@30996:   case (Suc N) then show ?case 
haftmann@30996:   apply (auto simp add: power_less_power_Suc less_Suc_eq)
haftmann@30996:   apply (subgoal_tac "1 * a^n < a * a^N", simp)
haftmann@30996:   apply (rule mult_strict_mono) apply (auto simp add: less_trans [OF zero_less_one] less_imp_le)
haftmann@30996:   done
haftmann@30996: qed
paulson@14348: 
nipkow@25134: lemma power_increasing_iff [simp]:
haftmann@30996:   "1 < b \<Longrightarrow> b ^ x \<le> b ^ y \<longleftrightarrow> x \<le> y"
haftmann@30996:   by (blast intro: power_le_imp_le_exp power_increasing less_imp_le)
paulson@15066: 
paulson@15066: lemma power_strict_increasing_iff [simp]:
haftmann@30996:   "1 < b \<Longrightarrow> b ^ x < b ^ y \<longleftrightarrow> x < y"
nipkow@25134: by (blast intro: power_less_imp_less_exp power_strict_increasing) 
paulson@15066: 
paulson@14348: lemma power_le_imp_le_base:
haftmann@30996:   assumes le: "a ^ Suc n \<le> b ^ Suc n"
haftmann@30996:     and ynonneg: "0 \<le> b"
haftmann@30996:   shows "a \<le> b"
nipkow@25134: proof (rule ccontr)
nipkow@25134:   assume "~ a \<le> b"
nipkow@25134:   then have "b < a" by (simp only: linorder_not_le)
nipkow@25134:   then have "b ^ Suc n < a ^ Suc n"
wenzelm@41550:     by (simp only: assms power_strict_mono)
haftmann@30996:   from le and this show False
nipkow@25134:     by (simp add: linorder_not_less [symmetric])
nipkow@25134: qed
wenzelm@14577: 
huffman@22853: lemma power_less_imp_less_base:
huffman@22853:   assumes less: "a ^ n < b ^ n"
huffman@22853:   assumes nonneg: "0 \<le> b"
huffman@22853:   shows "a < b"
huffman@22853: proof (rule contrapos_pp [OF less])
huffman@22853:   assume "~ a < b"
huffman@22853:   hence "b \<le> a" by (simp only: linorder_not_less)
huffman@22853:   hence "b ^ n \<le> a ^ n" using nonneg by (rule power_mono)
haftmann@30996:   thus "\<not> a ^ n < b ^ n" by (simp only: linorder_not_less)
huffman@22853: qed
huffman@22853: 
paulson@14348: lemma power_inject_base:
haftmann@30996:   "a ^ Suc n = b ^ Suc n \<Longrightarrow> 0 \<le> a \<Longrightarrow> 0 \<le> b \<Longrightarrow> a = b"
haftmann@30996: by (blast intro: power_le_imp_le_base antisym eq_refl sym)
paulson@14348: 
huffman@22955: lemma power_eq_imp_eq_base:
haftmann@30996:   "a ^ n = b ^ n \<Longrightarrow> 0 \<le> a \<Longrightarrow> 0 \<le> b \<Longrightarrow> 0 < n \<Longrightarrow> a = b"
haftmann@30996:   by (cases n) (simp_all del: power_Suc, rule power_inject_base)
huffman@22955: 
huffman@47192: lemma power2_le_imp_le:
wenzelm@53015:   "x\<^sup>2 \<le> y\<^sup>2 \<Longrightarrow> 0 \<le> y \<Longrightarrow> x \<le> y"
huffman@47192:   unfolding numeral_2_eq_2 by (rule power_le_imp_le_base)
huffman@47192: 
huffman@47192: lemma power2_less_imp_less:
wenzelm@53015:   "x\<^sup>2 < y\<^sup>2 \<Longrightarrow> 0 \<le> y \<Longrightarrow> x < y"
huffman@47192:   by (rule power_less_imp_less_base)
huffman@47192: 
huffman@47192: lemma power2_eq_imp_eq:
wenzelm@53015:   "x\<^sup>2 = y\<^sup>2 \<Longrightarrow> 0 \<le> x \<Longrightarrow> 0 \<le> y \<Longrightarrow> x = y"
huffman@47192:   unfolding numeral_2_eq_2 by (erule (2) power_eq_imp_eq_base) simp
huffman@47192: 
huffman@47192: end
huffman@47192: 
huffman@47192: context linordered_ring_strict
huffman@47192: begin
huffman@47192: 
huffman@47192: lemma sum_squares_eq_zero_iff:
huffman@47192:   "x * x + y * y = 0 \<longleftrightarrow> x = 0 \<and> y = 0"
huffman@47192:   by (simp add: add_nonneg_eq_0_iff)
huffman@47192: 
huffman@47192: lemma sum_squares_le_zero_iff:
huffman@47192:   "x * x + y * y \<le> 0 \<longleftrightarrow> x = 0 \<and> y = 0"
huffman@47192:   by (simp add: le_less not_sum_squares_lt_zero sum_squares_eq_zero_iff)
huffman@47192: 
huffman@47192: lemma sum_squares_gt_zero_iff:
huffman@47192:   "0 < x * x + y * y \<longleftrightarrow> x \<noteq> 0 \<or> y \<noteq> 0"
huffman@47192:   by (simp add: not_le [symmetric] sum_squares_le_zero_iff)
huffman@47192: 
haftmann@30996: end
haftmann@30996: 
haftmann@35028: context linordered_idom
haftmann@30996: begin
huffman@29978: 
haftmann@30996: lemma power_abs:
haftmann@30996:   "abs (a ^ n) = abs a ^ n"
haftmann@30996:   by (induct n) (auto simp add: abs_mult)
haftmann@30996: 
haftmann@30996: lemma abs_power_minus [simp]:
haftmann@30996:   "abs ((-a) ^ n) = abs (a ^ n)"
huffman@35216:   by (simp add: power_abs)
haftmann@30996: 
blanchet@35828: lemma zero_less_power_abs_iff [simp, no_atp]:
haftmann@30996:   "0 < abs a ^ n \<longleftrightarrow> a \<noteq> 0 \<or> n = 0"
haftmann@30996: proof (induct n)
haftmann@30996:   case 0 show ?case by simp
haftmann@30996: next
haftmann@30996:   case (Suc n) show ?case by (auto simp add: Suc zero_less_mult_iff)
huffman@29978: qed
huffman@29978: 
haftmann@30996: lemma zero_le_power_abs [simp]:
haftmann@30996:   "0 \<le> abs a ^ n"
haftmann@30996:   by (rule zero_le_power [OF abs_ge_zero])
haftmann@30996: 
huffman@47192: lemma zero_le_power2 [simp]:
wenzelm@53015:   "0 \<le> a\<^sup>2"
huffman@47192:   by (simp add: power2_eq_square)
huffman@47192: 
huffman@47192: lemma zero_less_power2 [simp]:
wenzelm@53015:   "0 < a\<^sup>2 \<longleftrightarrow> a \<noteq> 0"
huffman@47192:   by (force simp add: power2_eq_square zero_less_mult_iff linorder_neq_iff)
huffman@47192: 
huffman@47192: lemma power2_less_0 [simp]:
wenzelm@53015:   "\<not> a\<^sup>2 < 0"
huffman@47192:   by (force simp add: power2_eq_square mult_less_0_iff)
huffman@47192: 
huffman@47192: lemma abs_power2 [simp]:
wenzelm@53015:   "abs (a\<^sup>2) = a\<^sup>2"
huffman@47192:   by (simp add: power2_eq_square abs_mult abs_mult_self)
huffman@47192: 
huffman@47192: lemma power2_abs [simp]:
wenzelm@53015:   "(abs a)\<^sup>2 = a\<^sup>2"
huffman@47192:   by (simp add: power2_eq_square abs_mult_self)
huffman@47192: 
huffman@47192: lemma odd_power_less_zero:
huffman@47192:   "a < 0 \<Longrightarrow> a ^ Suc (2*n) < 0"
huffman@47192: proof (induct n)
huffman@47192:   case 0
huffman@47192:   then show ?case by simp
huffman@47192: next
huffman@47192:   case (Suc n)
huffman@47192:   have "a ^ Suc (2 * Suc n) = (a*a) * a ^ Suc(2*n)"
huffman@47192:     by (simp add: mult_ac power_add power2_eq_square)
huffman@47192:   thus ?case
huffman@47192:     by (simp del: power_Suc add: Suc mult_less_0_iff mult_neg_neg)
huffman@47192: qed
haftmann@30996: 
huffman@47192: lemma odd_0_le_power_imp_0_le:
huffman@47192:   "0 \<le> a ^ Suc (2*n) \<Longrightarrow> 0 \<le> a"
huffman@47192:   using odd_power_less_zero [of a n]
huffman@47192:     by (force simp add: linorder_not_less [symmetric]) 
huffman@47192: 
huffman@47192: lemma zero_le_even_power'[simp]:
huffman@47192:   "0 \<le> a ^ (2*n)"
huffman@47192: proof (induct n)
huffman@47192:   case 0
huffman@47192:     show ?case by simp
huffman@47192: next
huffman@47192:   case (Suc n)
huffman@47192:     have "a ^ (2 * Suc n) = (a*a) * a ^ (2*n)" 
huffman@47192:       by (simp add: mult_ac power_add power2_eq_square)
huffman@47192:     thus ?case
huffman@47192:       by (simp add: Suc zero_le_mult_iff)
huffman@47192: qed
haftmann@30996: 
huffman@47192: lemma sum_power2_ge_zero:
wenzelm@53015:   "0 \<le> x\<^sup>2 + y\<^sup>2"
huffman@47192:   by (intro add_nonneg_nonneg zero_le_power2)
huffman@47192: 
huffman@47192: lemma not_sum_power2_lt_zero:
wenzelm@53015:   "\<not> x\<^sup>2 + y\<^sup>2 < 0"
huffman@47192:   unfolding not_less by (rule sum_power2_ge_zero)
huffman@47192: 
huffman@47192: lemma sum_power2_eq_zero_iff:
wenzelm@53015:   "x\<^sup>2 + y\<^sup>2 = 0 \<longleftrightarrow> x = 0 \<and> y = 0"
huffman@47192:   unfolding power2_eq_square by (simp add: add_nonneg_eq_0_iff)
huffman@47192: 
huffman@47192: lemma sum_power2_le_zero_iff:
wenzelm@53015:   "x\<^sup>2 + y\<^sup>2 \<le> 0 \<longleftrightarrow> x = 0 \<and> y = 0"
huffman@47192:   by (simp add: le_less sum_power2_eq_zero_iff not_sum_power2_lt_zero)
huffman@47192: 
huffman@47192: lemma sum_power2_gt_zero_iff:
wenzelm@53015:   "0 < x\<^sup>2 + y\<^sup>2 \<longleftrightarrow> x \<noteq> 0 \<or> y \<noteq> 0"
huffman@47192:   unfolding not_le [symmetric] by (simp add: sum_power2_le_zero_iff)
haftmann@30996: 
haftmann@30996: end
haftmann@30996: 
huffman@29978: 
huffman@47192: subsection {* Miscellaneous rules *}
paulson@14348: 
huffman@47255: lemma power_eq_if: "p ^ m = (if m=0 then 1 else p * (p ^ (m - 1)))"
huffman@47255:   unfolding One_nat_def by (cases m) simp_all
huffman@47255: 
huffman@47192: lemma power2_sum:
huffman@47192:   fixes x y :: "'a::comm_semiring_1"
wenzelm@53015:   shows "(x + y)\<^sup>2 = x\<^sup>2 + y\<^sup>2 + 2 * x * y"
huffman@47192:   by (simp add: algebra_simps power2_eq_square mult_2_right)
haftmann@30996: 
huffman@47192: lemma power2_diff:
huffman@47192:   fixes x y :: "'a::comm_ring_1"
wenzelm@53015:   shows "(x - y)\<^sup>2 = x\<^sup>2 + y\<^sup>2 - 2 * x * y"
huffman@47192:   by (simp add: ring_distribs power2_eq_square mult_2) (rule mult_commute)
haftmann@30996: 
haftmann@30996: lemma power_0_Suc [simp]:
haftmann@30996:   "(0::'a::{power, semiring_0}) ^ Suc n = 0"
haftmann@30996:   by simp
nipkow@30313: 
haftmann@30996: text{*It looks plausible as a simprule, but its effect can be strange.*}
haftmann@30996: lemma power_0_left:
haftmann@30996:   "0 ^ n = (if n = 0 then 1 else (0::'a::{power, semiring_0}))"
haftmann@30996:   by (induct n) simp_all
haftmann@30996: 
haftmann@30996: lemma power_eq_0_iff [simp]:
haftmann@30996:   "a ^ n = 0 \<longleftrightarrow>
haftmann@30996:      a = (0::'a::{mult_zero,zero_neq_one,no_zero_divisors,power}) \<and> n \<noteq> 0"
haftmann@30996:   by (induct n)
haftmann@30996:     (auto simp add: no_zero_divisors elim: contrapos_pp)
haftmann@30996: 
haftmann@36409: lemma (in field) power_diff:
haftmann@30996:   assumes nz: "a \<noteq> 0"
haftmann@30996:   shows "n \<le> m \<Longrightarrow> a ^ (m - n) = a ^ m / a ^ n"
haftmann@36409:   by (induct m n rule: diff_induct) (simp_all add: nz field_power_not_zero)
nipkow@30313: 
haftmann@30996: text{*Perhaps these should be simprules.*}
haftmann@30996: lemma power_inverse:
haftmann@36409:   fixes a :: "'a::division_ring_inverse_zero"
haftmann@36409:   shows "inverse (a ^ n) = inverse a ^ n"
haftmann@30996: apply (cases "a = 0")
haftmann@30996: apply (simp add: power_0_left)
haftmann@30996: apply (simp add: nonzero_power_inverse)
haftmann@30996: done (* TODO: reorient or rename to inverse_power *)
haftmann@30996: 
haftmann@30996: lemma power_one_over:
haftmann@36409:   "1 / (a::'a::{field_inverse_zero, power}) ^ n =  (1 / a) ^ n"
haftmann@30996:   by (simp add: divide_inverse) (rule power_inverse)
haftmann@30996: 
haftmann@30996: lemma power_divide:
haftmann@36409:   "(a / b) ^ n = (a::'a::field_inverse_zero) ^ n / b ^ n"
haftmann@30996: apply (cases "b = 0")
haftmann@30996: apply (simp add: power_0_left)
haftmann@30996: apply (rule nonzero_power_divide)
haftmann@30996: apply assumption
nipkow@30313: done
nipkow@30313: 
huffman@47255: text {* Simprules for comparisons where common factors can be cancelled. *}
huffman@47255: 
huffman@47255: lemmas zero_compare_simps =
huffman@47255:     add_strict_increasing add_strict_increasing2 add_increasing
huffman@47255:     zero_le_mult_iff zero_le_divide_iff 
huffman@47255:     zero_less_mult_iff zero_less_divide_iff 
huffman@47255:     mult_le_0_iff divide_le_0_iff 
huffman@47255:     mult_less_0_iff divide_less_0_iff 
huffman@47255:     zero_le_power2 power2_less_0
huffman@47255: 
nipkow@30313: 
haftmann@30960: subsection {* Exponentiation for the Natural Numbers *}
wenzelm@14577: 
haftmann@30996: lemma nat_one_le_power [simp]:
haftmann@30996:   "Suc 0 \<le> i \<Longrightarrow> Suc 0 \<le> i ^ n"
haftmann@30996:   by (rule one_le_power [of i n, unfolded One_nat_def])
huffman@23305: 
haftmann@30996: lemma nat_zero_less_power_iff [simp]:
haftmann@30996:   "x ^ n > 0 \<longleftrightarrow> x > (0::nat) \<or> n = 0"
haftmann@30996:   by (induct n) auto
paulson@14348: 
nipkow@30056: lemma nat_power_eq_Suc_0_iff [simp]: 
haftmann@30996:   "x ^ m = Suc 0 \<longleftrightarrow> m = 0 \<or> x = Suc 0"
haftmann@30996:   by (induct m) auto
nipkow@30056: 
haftmann@30996: lemma power_Suc_0 [simp]:
haftmann@30996:   "Suc 0 ^ n = Suc 0"
haftmann@30996:   by simp
nipkow@30056: 
paulson@14348: text{*Valid for the naturals, but what if @{text"0<i<1"}?
paulson@14348: Premises cannot be weakened: consider the case where @{term "i=0"},
paulson@14348: @{term "m=1"} and @{term "n=0"}.*}
haftmann@21413: lemma nat_power_less_imp_less:
haftmann@21413:   assumes nonneg: "0 < (i\<Colon>nat)"
haftmann@30996:   assumes less: "i ^ m < i ^ n"
haftmann@21413:   shows "m < n"
haftmann@21413: proof (cases "i = 1")
haftmann@21413:   case True with less power_one [where 'a = nat] show ?thesis by simp
haftmann@21413: next
haftmann@21413:   case False with nonneg have "1 < i" by auto
haftmann@21413:   from power_strict_increasing_iff [OF this] less show ?thesis ..
haftmann@21413: qed
paulson@14348: 
haftmann@33274: lemma power_dvd_imp_le:
haftmann@33274:   "i ^ m dvd i ^ n \<Longrightarrow> (1::nat) < i \<Longrightarrow> m \<le> n"
haftmann@33274:   apply (rule power_le_imp_le_exp, assumption)
haftmann@33274:   apply (erule dvd_imp_le, simp)
haftmann@33274:   done
haftmann@33274: 
haftmann@51263: lemma power2_nat_le_eq_le:
haftmann@51263:   fixes m n :: nat
wenzelm@53015:   shows "m\<^sup>2 \<le> n\<^sup>2 \<longleftrightarrow> m \<le> n"
haftmann@51263:   by (auto intro: power2_le_imp_le power_mono)
haftmann@51263: 
haftmann@51263: lemma power2_nat_le_imp_le:
haftmann@51263:   fixes m n :: nat
wenzelm@53015:   assumes "m\<^sup>2 \<le> n"
haftmann@51263:   shows "m \<le> n"
haftmann@51263:   using assms by (cases m) (simp_all add: power2_eq_square)
haftmann@51263: 
haftmann@51263: 
haftmann@31155: 
haftmann@31155: subsection {* Code generator tweak *}
haftmann@31155: 
bulwahn@45231: lemma power_power_power [code]:
haftmann@31155:   "power = power.power (1::'a::{power}) (op *)"
haftmann@31155:   unfolding power_def power.power_def ..
haftmann@31155: 
haftmann@31155: declare power.power.simps [code]
haftmann@31155: 
haftmann@52435: code_identifier
haftmann@52435:   code_module Power \<rightharpoonup> (SML) Arith and (OCaml) Arith and (Haskell) Arith
haftmann@33364: 
paulson@3390: end
haftmann@49824: 




