The sequence of numbers which are sums of distinct powers of 4. The first few are 0, 1, 4, 5, 16, 17, 20, 21, 64, 65, 68, 69, 80, 81, 84, ... (Sloane's A000695). These numbers also satisfy the interesting properties that the sum of their binary digits equals the sum of their quaternary digits, and that they have identical representations in binary and negabinary.