A118027 a(0)=0. a(n) = a(n-1) + (smallest integer which is >= n and is missing from the earlier terms of the sequence).

0, 1, 3, 7, 11, 16, 22, 30, 38, 47, 57, 69, 81, 94, 108, 123, 140, 157, 175, 194, 214, 235, 258, 281, 305, 330, 356, 383, 411, 440, 471, 502, 534, 567, 601, 636, 672, 709, 748, 787, 827, 868, 910, 953, 997, 1042, 1088, 1136, 1184, 1233, 1283, 1334, 1386, 1439