This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A074938 #34 Jun 29 2025 21:37:07
%S A074938 1,3,9,13,27,31,37,39,81,85,91,93,109,111,117,121,243,247,253,255,271,
%T A074938 273,279,283,325,327,333,337,351,355,361,363,729,733,739,741,757,759,
%U A074938 765,769,811,813,819,823,837,841,847,849,973,975,981,985,999,1003,1009
%N A074938 Odd numbers such that base 3 representation contains no 2.
%C A074938 Odd numbers in A005836.
%C A074938 Numbers m such that coefficient of x^m equals -1 in Product_{k>=0} 1-x^(3^k).
%C A074938 Numbers k such that binomial(2k, k) == 2 (mod 3).
%C A074938 Sum of an odd number of distinct powers of 3. - _Emeric Deutsch_, Dec 03 2003
%H A074938 Amiram Eldar, <a href="/A074938/b074938.txt">Table of n, a(n) for n = 0..10000</a>
%H A074938 Emeric Deutsch and B. E. Sagan, <a href="https://arxiv.org/abs/math/0407326">Congruences for Catalan and Motzkin numbers and related sequences</a>, arXiv:math/0407326 [math.CO], 2004; J. Num. Theory 117 (2006), 191-215.
%F A074938 a(n) (mod 3) = A010059(n).
%F A074938 ((a(n)-1)/2) (mod 3) = A010060(n) = (1/2)*{binomial(2*a(n)+1, a(n)) (mod 3)}.
%t A074938 Select[Range[1,1111,2],Count[IntegerDigits[#,3],2]==0&] (* _Harvey P. Dale_, Dec 19 2010 *)
%o A074938 (Python)
%o A074938 def A074938(n): return int(bin((n<<1)+(n.bit_count()&1^1))[2:],3) # _Chai Wah Wu_, Jun 26 2025
%Y A074938 Intersection of A005408 and A005836.
%Y A074938 Cf. A006996, A074939.
%K A074938 easy,nonn
%O A074938 0,2
%A A074938 _Benoit Cloitre_, Oct 04 2002; Nov 15 2003