A180963 Numbers divisible by 3 with an odd number of 1s in their base 2 representation.

21, 42, 69, 81, 84, 87, 93, 117, 138, 162, 168, 171, 174, 186, 213, 234, 261, 273, 276, 279, 285, 309, 321, 324, 327, 333, 336, 339, 342, 345, 348, 351, 357, 369, 372, 375, 381, 405, 426, 453, 465, 468, 471, 477, 501, 522, 546, 552, 555, 558, 570, 597, 618

Offset: 1

Author

Dylan Hamilton, Sep 28 2010

Keywords

• nonn
• easy
• base

Comments

Not the same as multiples of 3 with a nonzero alternating binary digit sum: a(142) = 1368, but that sequence has 1365 as the 142nd term. - Charles R Greathouse IV, Feb 06 2015

Links

• Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

Crossrefs

Intersection of A000069 and A008585.

Cf. A036556.

Programs

Mathematica

Select[3 * Range[210], OddQ[DigitCount[#, 2, 1]] &] (* Amiram Eldar, Aug 06 2023 *)

PARI

isok(n) = (hammingweight(n) % 2) && (n % 3 == 0) \\ Michel Marcus, Jul 16 2013

Formula

a(n) = 3*A036556(n). - Amiram Eldar, Aug 06 2023