seqfan, I previously wrote : > fibonacci(n)=sum(binomial(n-k,k)) without any numerical check. It is incorrect. My corrected work : > C(n, k) = if(k>n,0,if(k<0,(-1)^(n+k)*C(-1-k,n-k),n!/k!/(n-k)!)) > ? oo=100;print(prod(n=-oo,oo,fibonacci(n+1)==sum(k=-oo,oo,C(n-k,k)))) > 1 which shows how careful you have to be. Shalom, Michael