>>57
>yx=1+py(x+1)+qy(x-1)+(1-p-q)yx
q(yx-y(x-1)+1/(p-q))=p(y(x+1)-yx+1/(p-q)) for p≠q
y(x+1)-yx+1/(p-q)=(q/p)^x(y1-y0+1/(p-q))
(a+b)/(p-q)=ya-y(a-1)+1/(p-q)+…+y(1-b)-y(-b)+1/(p-q)=((q/p)^(a-1)+…+(q/p)^(-b))(y1-y0+1/(p-q))
y0+b/(p-q)=y0-y(-1)+1/(p-q)+…+y(1-b)-y(-b)+1/(p-q)=((q/p)^(-1)+…+(q/p)^(-b))(y1-y0+1/(p-q))
(y0+b/(p-q))/((a+b)/(p-q)=((q/p)^(-1)+…+(q/p)^(-b))/((q/p)^(a-1)+…+(q/p)^(-b))
p(yx-y(x-1))=1+p(y(x+1)-yx) for p=q
y(x+1)-yx=y1-y0-x/p
0=ya-y(a-1)+…+y(1-b)-y(-b)=(a+b)(y1-y0)-((a-1)+…+(-b))/p
E(Dab)=y0=y0-y(-1)+…+y(1-b)-y(-b)=b(y1-y0)-((-1)+…+(-b))/p
=((a-1)+…+(-b))b/(a+b)p-((-1)+…+(-b))/p
=(b((a-1)+…+1)-a((-1)+…+(-b)))/(a+b)p