A043090 Every string of 2 consecutive base 4 digits contains exactly 2 distinct numbers.

0, 1, 2, 3, 4, 6, 7, 8, 9, 11, 12, 13, 14, 17, 18, 19, 24, 25, 27, 28, 29, 30, 33, 34, 35, 36, 38, 39, 44, 45, 46, 49, 50, 51, 52, 54, 55, 56, 57, 59, 68, 70, 71, 72, 73, 75, 76, 77, 78, 97, 98, 99, 100, 102, 103, 108, 109, 110, 113, 114, 115