# How to find functions & inputs whose output is a specific number

#### DaviFN

##### New member
I'm interested in the following problem: given a random number n (n can be gigantic), how do we find a pair function+input(s) whose output is n such that the input(s) are relatively small in size?

This problems arises in data compression; consider the bits that make up a file (or a substring of bits of the file) and treat it as a number (i.e. the bits are the binary representation of this number). If we could write a pair function+input(s) whose output happens to be the substring, this whole substring can be replaced by the function+input(s).

I've thought of expressing the number as sums (or differences) of relative big powers of prime numbers. Is this a good approach? And, if not, what would be a good one? And how to proceed?

Motivation of the question: A simples function like raising the nth prime number to a power S can result (depending on the values of p and S) on various outputs, each of which is unique (given that any number has only one prime factorization). If we pick p = 17 and S = 89435, for example, that's computationally trivial to compute (takes logarithmic time), and will result in a somewhat gigantic number. We can then generate a file whose bits are the same of the binary representation of this number (or at least some of the bits are). (This is just a rough example). The problem is going the other way: Given a bit string (hence, a number), how to express this specific bitstring with less bits (very few, actually) through a function that results in the number.