# [isabelle] Help with code

Hi all,


I am trying to get the following code2run but it comes up with an Illegal Command error at the 'hence' in the lemma double_sum_aux: and i wonder if you know why? I am new to Isabelle/Isar so not too sure how to make it work.

Rachel

theory CauchySchwarz
imports Complex_Main
begin

lemma real_sq: "(a::real)*a = a^2"
proof -
have "2 = Suc 1"
by simp
hence "a^2 = a^(Suc 1)"
apply -
apply (erule subst)
by simp
also have "\<dots> = a*(a^1)" by simp
also have "\<dots> = a*a" by simp
finally have "a^2 = a*a" by auto
thus ?thesis by simp
qed

lemma real_mult_wkorder:
fixes x::real
assumes xge0: "0 \<le> x" and yge0: "0 \<le> y"
shows "0 \<le> x*y"
proof cases
assume "x = 0"
thus ?thesis by simp
next
assume "x \<noteq> 0"
with xge0 have xgt0: "x > 0" by simp
show ?thesis
proof cases
assume "y = 0"
thus ?thesis by simp
next
assume "y \<noteq> 0"
with yge0 have "y > 0" by simp
with xgt0 have "x*y > 0" by (rule real_mult_order)
thus ?thesis by simp
qed
qed

lemma real_mult_order2:
fixes c::real
assumes c0: "0 \<le> c"
and xy: "x \<le> y"
shows "c*x \<le> c*y"
proof cases
assume "0 = c"
thus ?thesis by auto
next
assume "0 \<noteq> c"
with c0 have "0 < c" by simp
with xy show ?thesis by simp
qed

lemma real_sq_order:
fixes x::real
assumes xgt0: "0 \<le> x" and ygt0: "0 \<le> y" and sq: "x^2 \<le> y^2"
shows "x \<le> y"
proof (rule ccontr)
assume "\<not>(x \<le> y)"
then have ylx: "y < x" by simp
hence "y \<le> x" by simp
with xgt0 have "x*y \<le> x*x" by (rule real_mult_order2)
moreover
have "\<not>(y = 0)"
proof
assume "y = 0"
with ylx have xg0: "0 < x" by simp
from xg0 xg0 have "0 < x*x" by (rule real_mult_order)
moreover have "y*y = 0" by simp
ultimately show False using sq by auto
qed
with ygt0 have "0 < y" by simp
with ylx have "y*y < x*y" by auto
ultimately have "y*y < x*x" by simp
with sq show False by (auto simp add: real_sq)
qed

fixes x::real
shows "x*(y+z) = x*y + x*z"
proof -
have "x*(y+z) = (y+z)*x" by simp
also have "\<dots> = y*x + z*x" by (rule real_add_mult_distrib)
also have "\<dots> = x*y + x*z" by simp
finally show ?thesis .
qed

fixes x::real
shows "(x+y)*(z+w) = x*z + x*w + y*z + y*w"
proof -
have "(x+y)*(z+w) = x*(z+w) + y*(z+w)" by (rule real_add_mult_distrib)

also have "\<dots> = x*z + x*w + y*z + y*w" by (simp add: real_add_mult_distrib2)
  finally show ?thesis by simp
qed

lemma real_sub_mult_distrib_ex:
fixes x::real
shows "(x-y)*(z-w) = x*z - x*w - y*z + y*w"
proof -
have zw: "(z-w) = (z+ -w)" by simp
have "(x-y)*(z-w) = (x+ -y)*(z-w)" by simp
also have "\<dots> = x*(z-w) + -y*(z-w)" by (rule real_add_mult_distrib)
also from zw have "\<dots> = x*(z+ -w) + -y*(z+ -w)"
apply -
apply (erule subst)
by simp

also have "\<dots> = x*z + x*-w + -y*z + -y*-w" by (simp add: real_add_mult_distrib2)
  finally show ?thesis by simp
qed

lemma double_sum_expand:
fixes f::"nat \<Rightarrow> real"

shows "(\<Sum>j\<in>{1..n}. f j)*(\<Sum>j\<in>{1..n}. g j) = (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}.f k*g j))"
proof (induct n)
case 0 show ?case by simp
next
case (Suc n) {
assume nz: "n = 0"
then have

"(\<Sum>j\<in>{1..Suc n}. f j)*(\<Sum>j\<in>{1..Suc n}. g j) = f 1 * g 1" by simp
    moreover from nz have

"(\<Sum>k\<in>{1..Suc n}.(\<Sum>j\<in>{1..Suc n}.(f k)*(g j))) = f 1 * g 1" by auto
    ultimately have ?case by simp
}
moreover {
assume "\<not>(n = 0)"
then have nge1: "n \<ge> 1" by simp
hence sn: "Suc n > 1" by simp

let ?f = "\<Sum>j\<in>{1..n}. f j" and ?g = "\<Sum>j\<in>{1..n}. g j"


from sn setsum_cl_ivl_Suc have "(\<Sum>j\<in>{1..Suc n}. f j) = ?f + f (Suc n)" by auto
  moreover

from sn setsum_cl_ivl_Suc have "(\<Sum>j\<in>{1..Suc n}. g j) = ?g + g (Suc n)" by auto
  ultimately have

"(\<Sum>j\<in>{1..Suc n}. f j)*(\<Sum>j\<in>{1..Suc n}. g j) = (?f + f (Suc n))*(?g + g (Suc n))" by simp
  also have

"\<dots> = ?f * ?g + ?f*(g (Suc n)) + (f (Suc n))*?g + (f (Suc n))*(g (Suc n))"
    by (auto simp add: real_add_mult_distrib_ex)
also have

"\<dots> = (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}.(f k)*(g j))) + ?f*(g (Suc n)) + (f (Suc n))*?g + (f (Suc n))*(g (Suc n))" using Suc by simp
  also have

"\<dots> = (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}.(f k)*(g j))) + (\<Sum>j\<in>{1..n}. f j * g (Suc n)) + ?g * (f (Suc n)) + (f (Suc n))*(g (Suc n))" by (auto simp add: setsum_mult real_mult_commute)
  also have

"\<dots> = (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}.(f k)*(g j)) + (f k * g (Suc n))) + (?g * f (Suc n)) + (f (Suc n))*(g (Suc n))" by (auto simp add: setsum_addf [symmetric])
  also have

"\<dots> = (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..(Suc n)}.(f k)*(g j))) + (\<Sum>j\<in>{1..n}. g j)*(f (Suc n)) + g(Suc n)*(f (Suc n))" by simp
  also have

"\<dots> = (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..(Suc n)}.(f k)*(g j))) + (\<Sum>j\<in>{1..n}.g j * f (Suc n)) + g(Suc n)*(f (Suc n))" by (auto simp add: setsum_mult real_mult_commute)
  also from sn setsum_cl_ivl_Suc have

"\<dots> = (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..(Suc n)}.(f k)*(g j))) + (\<Sum>j\<in>{1..Suc n}.(g j)*(f (Suc n)))" by auto
  also from sn setsum_cl_ivl_Suc have

"\<dots> = (\<Sum>k\<in>{1..Suc n}.(\<Sum>j\<in>{1..(Suc n)}.(f k)*(g j)))" by (auto simp add: real_mult_commute)
  finally have ?case .
}
ultimately show ?case by (rule case_split_thm)
qed

lemma real_sq_exp:
fixes x::real
shows "(x*y)^2 = x^2 * y^2"
proof -
have "(x*y)^2 = (x*y)*(x*y)" by (simp add: real_sq)
also have "\<dots> = (x*x)*(y*y)" by simp
also have "\<dots> = x^2 * y^2" by (simp add: real_sq)
finally show ?thesis .
qed

lemma real_diff_exp:
fixes x::real
shows "(x-y)^2 = x^2 + y^2 - 2*x*y"
proof -
have "(x-y)^2 = (x-y)*(x-y)" by (simp only: real_sq)

also from real_sub_mult_distrib_ex have "\<dots> = x*x - x*y - y*x + y*y" by simp
  finally show ?thesis by (auto simp add: real_sq)
qed

lemma double_sum_equiv:
fixes f::"nat \<Rightarrow> real"

shows "(\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}. f k * g j)) = (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}. f j * g k))"
proof -

have "(\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}. (f k)*(g j))) = (\<Sum>j\<in>{1..n}.f j)*(\<Sum>j\<in>{1..n}.g j)" by (simp only: double_sum_expand) also have "\<dots> = (\<Sum>j\<in>{1..n}. g j)*(\<Sum>j\<in>{1..n}. f j)" by simp also have "\<dots> = (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}. g k * f j))" by (simp only: double_sum_expand)
  finally show ?thesis by (simp add: real_mult_commute)
qed


text {* Theorem: Take $V$ to be some vector space possessing a norm and inner product, then for all $a,b \in V$ the following inequality holds: @{text "|a.b| \<le> \<parallel>a\<parallel> * \<parallel>b\<parallel>"}. Specifically, in the Real case, the norm is in the Euclidean length and the inner product is the standard dot product. *}

types vector = "(nat \<Rightarrow> real)*nat";

constdefs ith :: "vector \<Rightarrow> nat \<Rightarrow> real"
"ith v i \<equiv> (fst v) i"
vlen :: "vector \<Rightarrow> nat"
"vlen v \<equiv> (snd v)"

constdefs dot :: "vector \<Rightarrow> vector \<Rightarrow> real"
"dot a b \<equiv> (\<Sum>j\<in>{1..(vlen a)}.(ith a j)*(ith b j))"
norm :: "vector \<Rightarrow> real"
"norm v \<equiv> sqrt (\<Sum>j\<in>{1..(vlen v)}.(ith v j)^2)"

syntax(HTML output)

"norm" :: "vector \<Rightarrow> real"

lemma norm_dot: "norm(v)=sqrt(dot v v)"
proof -

have "sqrt(dot v v) = sqrt(\<Sum>j\<in>{1..(vlen v)}. (ith v j)*(ith v j))" by (unfold dot_def, simp) also with real_sq have "\<dots> = sqrt(\<Sum>j\<in>{1..(vlen v)}.(ith v j)^2)" by simp
  also have "\<dots> = norm v" by (unfold norm_def, simp)
finally show ?thesis ..
qed

lemma norm_pos: "norm v  \<ge> 0"
proof -
have "\<forall>j. (ith v j)^2 \<ge> 0" by (unfold ith_def, auto)
hence "\<forall>j\<in>{1..(vlen v)}. (ith v j)^2 \<ge> 0" by simp
with setsum_nonneg have "(\<Sum>j\<in>{1..(vlen v)}. (ith v j)^2) \<ge> 0" .

with real_sqrt_ge_zero have "sqrt(\<Sum>j\<in>{1..(vlen v)}. (ith v j)^2) \<ge> 0" .
  thus ?thesis by (unfold norm_def)
qed

lemma double_sum_aux:
fixes f::"nat \<Rightarrow> real"

shows "(\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}. f k * g j)) = (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}.(f k * g j + f j * g k)/2))"
proof -

have "2 * (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}. f k * g j)) = (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}. f k * g j)) + (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}. f k * g j))" by simp also have "\<dots> = (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}. f k * g j)) + (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}. f j * g k))" by (simp only: double_sum_equiv) also have "\<dots> = (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}. f k * g j + f j * g k))" by (auto simp add: setsum_addf) finally have "2 * (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}. f k * g j)) = (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}. f k * g j + f j * g k))" hence "(\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}. f k * g j)) = (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}.(f k * g j + f j * g k)))*(1/2)" by auto also have "\<dots> = (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}.(f k * g j + f j* g k)*(1/2)))" by (simp add: setsum_mult real_mult_commute)
  finally show ?thesis by (auto simp add: inverse_eq_divide)
qed

theorem CauchySchwarzReal:
fixes x::vector
assumes "vlen x = vlen y"
shows "|dot x y| \<le> (norm x)*(norm y)"
proof -
have "0 \<le> |dot x y|" by simp

moreover have "0 \<le> (norm x)*(norm y)" by (auto simp add: norm_pos intro: mult_nonneg_nonneg)
  moreover have "|dot x y|^2 \<le> ((norm x)*(norm y))^2"
proof -
txt {* We can rewrite the goal in the following form \<dots> *}
have "((norm x)*(norm y))^2 - |dot x y|^2 \<ge> 0"
proof -
obtain n where nx: "n = vlen x" by simp
hence ny: "n = vlen y" by simp {
txt {* Some preliminary simplification rules. *}
have "\<forall>j\<in>{1..n}. (ith x j)^2 \<ge> 0" by simp

hence "(\<Sum>j\<in>{1..n}. (ith x j)^2) \<ge> 0" by (rule setseum_nonneg) hence xp: "sqrt (\<Sum>j\<in>{1..n}.(ith x j)^2 = (\<Sum>j\<in>{1..n}.(ith x j)^2)" by (rule real_sqrt_pow2)

have "\<forall>j\<in>{1..n}.(ith y j)^2 \<ge> 0" by simp
hence "(\<Sum>j\<in>{1..n}.(ith y j)^2) \<ge> 0" by (rule setsum_nonneg)

hence yp: "(sqrt (\<Sum>j\<in>{1..n}.(ith y j)^2))^2 = (\<Sum>j\<in>{1..n}.(ith y j)^2)" by (rule real_sqrt_pow2)


txt {* The main result of this section is that @{text "(\<parallel>x\<parallel>*\<parallel>y\<parallel>)^2"} can be written as a double sum. *} have "((norm x)*(norm y))^2 = (norm x)^2 * (norm y)^2" by (simp add: real_sq_exp) also from nx ny have "\<dots> = (sqrt (\<Sum>j\<in>{1..n}.(ith x j)^2))^2 * (sqrt (\<Sum>j\<in>{1..n}.(ith y j)^2))^2" by (unfold norm_def, auto) also from xp yp have "\<dots> = (\<Sum>j\<in>{1..n}.(ith x j)^2)*(\<Sum>j\<in>{1..n}.(ith y j)^2)" by simp also from double_sum_expand have "\<dots> = (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}.(ith x k)^2)*(ith y j)^2)))" . finally have "((norm x)*(norm y))^2 = (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}.(ith x k)^2)*(ith y j)^2)))"
    }
moreover {

txt {* We also show that @{text "|x.y|^2"} can be expressed as a double sum. *}
      have "|dot x y|^2 = (dot x y)^2" by simp

also from nx have "\<dots> = (\<Sum>j\<in>{1..n}.(ith x j)*(ith y j))^2" by (unfold dot_def, simp) also from real_sq have "\<dots> = (\<Sum>j\<in>{1..n}. (ith x j)*(ith y j))*(\<Sum>j\<in>{1..n}. (ith x j)*(ith y j))" by simp also from double_sum_expand have "\<dots> = (\<Sum>k\<in>{1..n}. (\<Sum>j\<in>{1..n}. ((ith x k)*(ith y k))*((ith x j)*(ith y j))))" by simp finally have "|dot x y|^2 = (\<Sum>k\<in>{1..n}. (\<Sum>j\<in>{1..n}. ((ith x k)*(ith y k))*((ith x j)*(ith y j))))" .
    }

txt {* We now manipulate the double sum expressions to get the required inequality. *} ultimately have "((norm x)*(norm y))^2 - |x.y|^2 = (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}. ((ith x k)^2)*((ith y j)^2))) - (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}.((ith x k)*(ith y k))*((ith x j)*(ith y j))))" by simp also have "\<dots> = (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}. (((ith x k)^2*(ith y j)^2) + ((ith x j)^2*(ith y k)^2))/2)) - (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}. ((ith x k)*(ith y k))*((ith x j)*(ith y j))))" by (simp only: double_sum_aux) also have "\<dots> = (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}. (((ith x k)^2*(ith y j)^2) + ((ith x j)^2*(ith y k)^2))/2 - ((ith x k)*(ith y k))*((ith x j)*(ith y j))))" by (auto simp add: setsum_subtractf) also have "\<dots> = (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}.(inverse 2)*2*( (((ith x k)^2*(ith y j)^2) + ((ith x j)^2*(ith y k)^2))*(1/2) - ((ith x k)*(ith y k))*((ith x j)*(ith y j)))))" by auto also have "\<dots> = (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}.(inverse 2)*(2*( (((ith x k)^2*(ith y j)^2) + ((ith x j)^2*(ith y k)^2))*(1/2) - ((ith x k)*(ith y k))*((ith x j)*(ith y j)))))))" by (simp only: real-mult_assoc) also have "\<dots> = (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}.(inverse 2)*(( (((ith x k)^2*(ith y j)^2) + ((ith x j)^2*(ith y k)^2))*2*(inverse 2) - 2*((ith x k)*(ith y k))*((ith x j)*(ith y j))))))" by (auto simp add: real_add_mult_distrib real_mult_assoc mult_ac) also have "\<dots> = (\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}.(inverse 2)*((( (((ith x k)^2*(ith y j)^2) + ((ith x j)^2*(ith y k)^2))) - 2*((ith x k)*(ith y k))*((ith x j)*(ith y j)))))))" by (simp only: real_mult_assoc, simp) also have "\<dots> = (inverse 2)*(\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}.((((ith x k)^2*(ith y j)^2) + ((ith x j)^2*(ith y k)^2)) - 2*((ith x k)*(ith y k))*((ith x j)*(ith y j)))))" by (simp only: setseum_mult) also have "\<dots> = (inverse 2)*(\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}.((ith x k)*(ith y j) - (ith x j)*(ith y k))^2))" by (simp only: real_diff_exp real_sq_exp, auto simp add: mult_ac)
    also have "\<dots> \<ge> 0"
proof -
{
fix k::nat

have "\<forall>j\<in>{1..n}.((ith x k)*(ith y j) - (ith x j)*(ith y k))^2 \<ge> 0" by simp hence "(\<Sum>j\<in>{1..n}.((ith x k)*(ith y j) - (ith x j)*(ith y k))^2 \<ge> 0" by (rule setsum_nonneg)
      }

hence "\<forall>k\<in>{1..n}.(\<Sum>j\<in>{1..n}.((ith x k)*(ith y j) - (ith x j)*(ith y k))^2 \<ge> 0" by simp hence "(\<Sum>k\<in>{1..n}.(\<Sum>j\<in>{1..n}.((ith x k)*(ith y j) - (ith x j)*(ith y k))^2)) \<ge> 0" by (rule setsum_nonneg)
      thus ?thesis by simp
qed
finally show "((norm x)*(norm y))^2 - |dot x y|^2 \<ge> 0" .
qed
thus ?thesis by simp
qed
ultimately show ?thesis by (rule real_sq_order)
qed

end

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