>
Digits:=100;
Sum2N:=proc(N)
evalf(sum(n/(2^n),n=1..N));
end:
> Sum2N(400);
>
Sum2N2:=proc(N)
evalf(sum(n^2/(2^n),n=1..N));
end:
> Sum2N2(400);
>
Sum2N3:=proc(N)
evalf(sum(n^3/(2^n),n=1..N));
end:
> Sum2N3(400);
>