PARI/gp

[Xorter]

Hi, again!

I would like to algoritmize the fractional derivatives and integrals, but here are some problems.
This is my code:

Code:

int1(f,a,b,h)={return(sum(k=floor(min(a*h,b*h)),ceil(max(a*h,b*h)),sign(b-a)*subst(f,x,k/h)/h));}

Stein(f,n,a,h)={return(der1(int1(f,0,a,h),a,h)/gamma(1-n));}

So e. g. D^2.5 x^2.5 = gamma(2.5-1) = Stein(x^2.5,2.5,0,1000.0), but it returns with this error message:
domain error in gpow [irrational exponent]: valuation != 0

Furthermore my Stein function return with ~0 in other cases. Why?

EDIT:
Here is the original formula:
https://wikimedia.org/api/rest_v1/media/...cc722c1f49
Oops, it looks I forgot some things from the my algorithm. It will be checked soon.

EDIT2:
Here is the new algorithm which is not working, too:

Code:

Stein(f,n,a,h)={return(der1(int1(subst(f,x,u)/(x-u)^n,0,a,h),a,h)/gamma(1-n));}