SparseInteger

A value of this type represents a non-negative integer that can be very large (like those that occur as numeric values of high power towers, much larger than those that can be represented by BigInteger within feasible memory limits), subject to a condition that either:

• the number fits into ulong type verbatim

• the number of 1's in its binary form is a moderate number (such that an array of that size can be allocated), and each of the positions of 1's in its binary form is, recursively, a number that can be represented by SparseInteger.

Code originally taken from https://github.com/VladimirReshetnikov/Oeis.A002845