∫(0→1)dx∫(0→x) [(x-y)^(a-1)]f(y)dy=∫(0→1)dy∫(y→1) [(x-y)^(a-1)]f(y)dx，对x积分的原函数是f(y)×1/a(x-y)^a=∫(0→1) f(y)×1/a(1-y)^a dy，换元y=1-z=1/a∫(0→1）z^af(1-z) dz=1/a∫(0→1）y^af(1-y) dy
