Problem Statement

Repunits are numbers that consist entirely of repetitions of the digit one (unity). The notation Rn symbolizes the repunit made up of n ones. Every prime p larger than 5, evenly divides the repunit Rp-1.

The repunit R6 is evenly divisible by 7. 111111 / 7 = 15873 The repunit R42 is evenly divisible by 43. 111111111111111111111111111111111111111111 / 43 = 2583979328165374677002583979328165374677 And so on.

There are composite numbers that also have this same property. They are often referred to as deceptive non-primes or deceptive numbers.

The repunit R90 is evenly divisible by the composite number 91 (=7*13).

Let's start with the solution:

wheel=. 4 6 2 6 4 2 4 2
fermat=. {{1 = 10 (y&|@^) <: y}}"0

_10 ]\ (#~ fermat) (#~ 0&p:) +/\ 49 , 15e4 $ wheel