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 A375241 #10 Aug 16 2024 23:13:50
%S A375241 9,12,18,19,24,27,30,33,37,39,42,46,48,51,54,57,60,63,64,69,72,73,75,
%T A375241 81,82,84,87,90,93,96,99,100,102,108,109,111,114,117,118,123,126,127,
%U A375241 129,132,135,138,141,144,145,147,150,154,156,159,162,163,165,168,172
%N A375241 Nontriangular numbers with digital root in {1, 3, 6, 9}.
%C A375241 Except for 0, the triangular numbers (A000217) have digital root in {1, 3, 6, 9} (A055264), but the reverse is not always true since there are nontriangular numbers (A014132) with digital root in the same set.
%D A375241 Albert H. Beiler, Recreations in the theory of numbers, New York, Dover, (2nd ed.) 1966. See p. 190.
%H A375241 Stefano Spezia, <a href="/A375241/b375241.txt">Table of n, a(n) for n = 1..10000</a>
%t A375241 A010888[n_]:=If[n>0,n - 9*Floor[(n-1)/9],0]; Select[Range[0,200], !OddQ[Sqrt[1+8#]] && MemberQ[{1,3,6,9},A010888[#]] &]
%Y A375241 Intersection of A014132 and A055264. Complement of A375242 in A014132.
%Y A375241 Cf. A000217, A010888.
%K A375241 nonn,base,easy
%O A375241 1,1
%A A375241 _Stefano Spezia_, Aug 07 2024