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 A055264 #31 Aug 08 2024 08:33:54
%S A055264 0,1,3,6,9,10,12,15,18,19,21,24,27,28,30,33,36,37,39,42,45,46,48,51,
%T A055264 54,55,57,60,63,64,66,69,72,73,75,78,81,82,84,87,90,91,93,96,99,100,
%U A055264 102,105,108,109,111,114,117,118,120,123,126,127,129,132,135,136,138,141
%N A055264 Possible values of A055263; numbers equal to 0, 1, 3 or 6 modulo 9.
%C A055264 The terms are the possible digit sums of a triangular number. - _Amarnath Murthy_, Jan 09 2002
%D A055264 Albert H. Beiler, Recreations in the theory of numbers, New York, Dover, (2nd ed.) 1966. See p. 190.
%H A055264 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (1,0,0,1,-1).
%F A055264 a(n) = a(n-4) + 9 = 9*floor(n/4) + (n mod 4)*(1 + (n mod 4))/2.
%F A055264 G.f.: x*(1+2*x+3*x^2+3*x^3)/((1-x)^2*(1+x)*(1+x^2)). - _R. J. Mathar_, Sep 30 2008
%F A055264 E.g.f.: (3*cos(x) + (9*x - 3)*cosh(x) - sin(x) + (9*x - 4)*sinh(x))/4. - _Stefano Spezia_, Aug 07 2024
%t A055264 Select[Range[0,200],MemberQ[{0,1,3,6},Mod[#,9]]&] (* _Harvey P. Dale_, Apr 10 2014 *)
%t A055264 #+{0,1,3,6}&/@(9*Range[0,20])//Flatten (* _Harvey P. Dale_, Jun 03 2019 *)
%o A055264 (Python)
%o A055264 def A055264(n): return (0,1,3,6)[n&3]+9*(n>>2) # _Chai Wah Wu_, Jan 30 2023
%Y A055264 Cf. A055263.
%K A055264 easy,nonn,base
%O A055264 0,3
%A A055264 _Henry Bottomley_, May 08 2000