A044153 Numbers n such that string 2,1 occurs in the base 7 representation of n but not of n-1.

15, 64, 105, 113, 162, 211, 260, 309, 358, 407, 448, 456, 505, 554, 603, 652, 701, 735, 791, 799, 848, 897, 946, 995, 1044, 1093, 1134, 1142, 1191, 1240, 1289, 1338, 1387, 1436, 1477, 1485, 1534, 1583, 1632, 1681, 1730, 1779