# Convergence acceleration of a series by using optimal parameters

One of the ways of accelerating the convergence of a series is by transforming into a faster series using optimal parameters. Examples of this approach can be found in this paper. I obtained a generalization of this method to express a series in terms of two more independent variables:

If $$|\frac{x}{x+y}| < 1$$. Then,

$$\sum_{n = 0}^{\infty}a_nx^n = \Big(\frac{y}{x+y}\Big)^{r+1} \sum_{n=0}^{\infty}\Big(\frac{x}{x+y}\Big)^{n} \sum_{k=0}^{n} {n+r\choose k+r}a_k y^k$$

This expresses a power series in the LHS in terms of two independent variables $$y$$ and $$r$$ which in theory can be fine tuned to make the series converge faster.

Question: How to choose the optimal $$y$$ and $$r$$ so that the RHS converges at its fastest rate?

Note: Asked this in MSE but got no replies in a week. Posting it here and deleted it from MSE to avoid duplication.

• the right-hand-side of your equation depends on $a_0$, but the left-hand-side does not, how can that be? – Carlo Beenakker May 25 at 17:20