07-30-2017, 06:17 AM

A "pan-prime-digit number" is a natural number containing all four prime digits (2, 3, 5, and 7) in any order, one or more times each, but no other digits. The smallest pan-prime-digit number is 2357, which also happens to be a prime number, but there are obviously infinitely many pan-prime-digit numbers, and probably infinitely many prime ones. I'm pretty sure that the smallest pan-prime-digit number which is a perfect SQUARE is 23377225 (equal to 4835^2). There are probably infinitely many pan-prime-digit squares.

However, it is my hypothesis that there is ONLY ONE pan-prime-digit CUBE. I would be delighted beyond words if anybody could either prove (mathematically) or disprove (by counterexample) this hypothesis. Needless to say, finding the one known pan-prime-digit cube is left as a mini programming challenge. This posting appears in the "Not remotely" forum because no current HP programmable calculator is fast enough to find the number in a reasonable amount of time; it's surprisingly large.

However, it is my hypothesis that there is ONLY ONE pan-prime-digit CUBE. I would be delighted beyond words if anybody could either prove (mathematically) or disprove (by counterexample) this hypothesis. Needless to say, finding the one known pan-prime-digit cube is left as a mini programming challenge. This posting appears in the "Not remotely" forum because no current HP programmable calculator is fast enough to find the number in a reasonable amount of time; it's surprisingly large.