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 A067991 #9 Dec 30 2023 11:06:38
%S A067991 1,2,3,7,11,12,27,31,56,107,109,132,309,343,627,968,1332,1891,3129,
%T A067991 3434,5291,5656,8831,13332,18972,19492,28248,37067,58309,107516,
%U A067991 140547,278172,281743,368507,424256,774325,1247307,2788547,3126968,3660565,3949427,7732916
%N A067991 a(n) = k such that the k-th triangular number is A068808(n).
%C A067991 a(n) = k such that A000217(k) = A068808(n). - _Jon E. Schoenfield_, Dec 30 2023
%F A067991 a(n) = (sqrt(1 + 8*A068808(n)) - 1)/2. - _Jon E. Schoenfield_, Dec 30 2023
%e A067991 a(8) = 31 because A068808(n) = 496 = 31*(31+1)/2 = A000217(31).
%p A067991 dig := X->convert((convert(X,base,10)),`+`); T := k->k*(k+1)/2; S := k->seq(dig(T(i)),i=1..k-1); seq(`if`(dig(T(i))>max(S(i)),i,printf("")),i=1..600);
%Y A067991 Cf. A000217, A068808.
%K A067991 nonn
%O A067991 1,2
%A A067991 Francois Jooste (phukraut(AT)hotmail.com), Mar 10 2002
%E A067991 Better name and more terms from _Jon E. Schoenfield_, Dec 30 2023