Unvigesimal is a number system where the base is 21 and all numbers from 1 - 18 have a repetend of 4 or less. Full Repetend Prime Numbers: 19, 23, 29, 31, 53, 71, 97, 103, 107, 113, 137, 139, 149, 157, 179, 181, 191, 197, 223, 233, 239, 263, 271, 281, 307, 313, 317, 347, 359, 389, 397, 401, 409, 431
