- the digits of N are all unique
- t=i^3, f=j^4, and s=k^7 for some integers i,j,k
- t, f, and s are each at least 3 digits
- there can be overlap of some of the digits of each of the powers as they appear in N.
For example, if N is abcdefg, then a valid solution would be if abcd, cde, and defg were a perfect cube, a 4th power and a 7th power in some order.
Furthermore, if a number's digits are abcd, then it is included in xyabcdz but not included in xabycdz.