A043931 Numbers n such that 5 and 6 occur juxtaposed in the base 7 representation of n but not of n+1.

41, 47, 90, 96, 139, 145, 188, 194, 237, 243, 293, 335, 341, 384, 390, 433, 439, 482, 488, 531, 537, 580, 586, 636, 678, 684, 727, 733, 776, 782, 825, 831, 874, 880, 923, 929, 979, 1021, 1027, 1070, 1076, 1119, 1125, 1168, 1174