I tried the various suggestions and I think you hit the reason beneath it, when I put the * in b(b-1), everything works but when I left out the *, Maple failed to recognise. Thanks for your brilliant help! Andrew from Hong Kong

I tried the various suggestions and I think you hit the reason beneath it, when I put the * in b(b-1), everything works but when I left out the *, Maple failed to recognise. Thanks for your brilliant help! Andrew from Hong Kong