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