# Infinite cake

This week’s Riddler Express is a short problem about infinite series. Let’s dig in! (I paraphrased the question)

You and your infinitely many friends are sharing a cake, and you come up with several different methods of splitting it.

1. Friend 1 takes half of the cake, Friend 2 takes a third of what remains, Friend 3 takes a quarter of what remains after Friend 2, Friend 4 takes a fifth of what remains after Friend 3, and so on.
2. Friend 1 takes $1/2^2$ (or one-quarter) of the cake, Friend 2 takes $1/3^2$ (or one-ninth) of what remains, Friend 3 takes $1/4^2$ of what remains after Friend 3, and so on.
3. Same as previous, with even denominators only: Friend 1 takes $1/2^2$ of the cake, Friend 2 takes $1/4^2$ of what remains, Friend 3 takes $1/6^2$ of what remains after Friend 2, and so on.

For each of these methods, after your infinitely many friends take their respective pieces, how much cake is left?

Here is my solution
[Show Solution]

## One thought on “Infinite cake”

1. Guy D. Moore says:

Your solution to part 3 is very nice.
I found a rather clumsier approach: there is an explicit expression for x_n:
x_n = ( (2n)!)^2 (2n+1) / ( 2^(4n) (n!)^4 )
which you can prove by induction. We find the large-n limit using Stirling’s approximation.