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 A117978 #40 Aug 23 2025 17:14:44 %S A117978 0,6,28,66,406,666,820,2080,2628,8646,28680,42486,48828,64620,66066, %T A117978 80200,84666,200028,204480,228826,264628,288420,426426,446040,468028, %U A117978 484620,600060,626640,644680,686206,828828,886446,2222886,2248260,2862028,2888406 %N A117978 Triangular numbers with only even digits. %C A117978 There are infinitely many terms. For example all 88...88 6 44...44 6 with an equal number of 8's and 4's (their triangular indices being A185127). - _Shyam Sunder Gupta_, Aug 16 2025 %H A117978 Jon E. Schoenfield, <a href="/A117978/b117978.txt">Table of n, a(n) for n = 1..10000</a> (first 1000 terms from Harvey P. Dale) %H A117978 Shyam Sunder Gupta, <a href="https://doi.org/10.1007/978-981-97-2465-9_3">Triangular Numbers</a>, Exploring the Beauty of Fascinating Numbers, Springer (2025) Ch. 3, 82-125. %t A117978 Select[Accumulate[Range[0,5000]],Count[IntegerDigits[#],_?(OddQ)] ==0&] (* _Harvey P. Dale_, Oct 21 2011 *) %Y A117978 Intersection of A000217 and A014263. %Y A117978 Cf. A117960, A185127. %K A117978 base,nonn,changed %O A117978 1,2 %A A117978 Luc Stevens (lms022(AT)yahoo.com), May 03 2006 %E A117978 Corrected (a(32) was in error) and extended by _Harvey P. Dale_, Oct 21 2011