y=Σa[i]*x^i
x^2*y''=Σi(i-1)a[i]*x^i
xy'=Σia[i]*x^i
(x^2-n^2)y=Σ(a[i-2]-n^2a[i])*x^i
x^2*y''+xy'+(x^2-n^2)y=Σ(a[i-2]+(i^2-n^2)a[i])x^i
a[i-2]=(n^2-i^2)a[i]でいいのかking