A043451 Numbers n such that number of 7's in base 8 is 3.

511, 1023, 1535, 2047, 2559, 3071, 3583, 3647, 3711, 3775, 3839, 3903, 3967, 4031, 4039, 4047, 4055, 4063, 4071, 4079, 4087, 4088, 4089, 4090, 4091, 4092, 4093, 4094, 4607, 5119, 5631, 6143, 6655, 7167, 7679, 7743, 7807