PARI GP is pseudoprime

f(n, s)=my(t); forstep(k=n, 1, -1, t=s*t+k); t
a2200=f(2200,560890);


with a2200

the smallest integer s>n such that 1+2*s+3*s^2+...+n*s^{n-1} is prime

OEIS A217785

Zhi Wei SUN

To be completed...