A044451 Numbers n such that string 2,1 occurs in the base 4 representation of n but not of n+1.

9, 25, 39, 41, 57, 73, 89, 103, 105, 121, 137, 159, 167, 169, 185, 201, 217, 231, 233, 249, 265, 281, 295, 297, 313, 329, 345, 359, 361, 377, 393, 415, 423, 425, 441, 457, 473, 487, 489, 505, 521, 537, 551, 553, 569, 639, 649