You accidentally edited the edge cases to have a space.

This solution does not work anymore due to comparing a float with an integer of large size.
You can avoid this by removing the division on the left side and multiplying the right side by 4 instead.

You are trying to write to a variable outside the scope of your function with p0 += something.
(It's bad practice to use functions to edit variables outside of your function to begin with most of the time)

By declaring pop you have a variable you can write to and do what you want with inside your function.

Nevermind what I said

Why the +0.005 to my final answer you ask?
The answer: floating point arithmetic sucks, it's a work-around for rounding errors.
Unfortunately I do not know of any way to get rid of this, if anyone can fork this and provide a proper answer that would be nice.

And yes for the series f(x) = a * f(x-1) + b the general solution for x finding value Q:
log_a((Q+b/(a-1))/(f(0)+b/(a-1))) Maths baby