Strong Repunits

The number 7 is special, because 7 is 111 written in base 2, and 11 written in base 6 (i.e. 7₁₀ = 11₆ = 111₂). In other words, 7 is a repunit in at least two bases b > 1.

We shall call a positive integer with this property a strong repunit. It can be verified that there are 8 strong repunits below 50: {1,7,13,15,21,31,40,43}. Furthermore, the sum of all strong repunits below 1000 equals 15864.

Written by gamwe6 who lives and works in San Francisco building useful things. You should follow him on Twitter