Here is an interesting numerical means to compute the radius of convergence. It requires that the coefficients of x^n , alternate or are all the same sign.

We need to compute the ordered pairs:

The above generates:

Extrapolate the value in the second column for n = ∞. Looks like we get -.5625.

Here is another one:

This one is a little tougher we must extrapolate to 1 / ∞. So you do a linear fit of the last two points. You get:

The limit of that as x approaches 0 is obviously .2498259

Which is close to the the exact answer of 2.

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