>>398
a'Aa=Σ[k,j]a(k)A(k,j)a(j)

∂(a'Aa)/∂a の第 i 成分
= ∂/∂a(i) {Σ[k,j]a(k)A(k,j)b(j)}
= Σ[j]A(i,j)a(j) + Σ[k]a(k)A(k,i)
= Σ[j]{A(i,j)+A(j,i)}a(j)