Almost perfect number

Math An almost perfect number is a number which has a deficiency of 1. The almost perfect numbers are the powers of 2.

They are:

1, 2, 4, 8, 16, ...

This can be proved by the fact that the sum of the reciprocals of the powers of 2 is equal to 2.

\[\sum_{n=0}^k\frac{1}{2^n}=1\frac{2^k-1}{2^k}\]