i am stuck on this question for a while and i'm not sure about it
i need to calculate the radius of convergence for: $\sum _{n=1}^\infty d(n)x^n$
what i did:
since i cannot use the root test or check the next eleent against its previous(ratio test), i think that to find the radius of convergence i need to check for the upper limit(sup).
so what i did is this: if n=1 we get that its one, so while going to a very large n, i think that $d(n) < n$. if this is correct, then tha radius should be: $\frac{1}{R}=\limsup_{n \to \infty}d(n) ^ {\frac{1}{n}}$. however, i don't know how to calculate it.
am i correct? how can i calculate the radius of convergence here?