A067991 a(n) = k such that the k-th triangular number is A068808(n).

1, 2, 3, 7, 11, 12, 27, 31, 56, 107, 109, 132, 309, 343, 627, 968, 1332, 1891, 3129, 3434, 5291, 5656, 8831, 13332, 18972, 19492, 28248, 37067, 58309, 107516, 140547, 278172, 281743, 368507, 424256, 774325, 1247307, 2788547, 3126968, 3660565, 3949427, 7732916

Offset: 1

Author

Francois Jooste (phukraut(AT)hotmail.com), Mar 10 2002

Keywords

• nonn

Comments

a(n) = k such that A000217(k) = A068808(n). - Jon E. Schoenfield, Dec 30 2023

Examples

			a(8) = 31 because A068808(n) = 496 = 31*(31+1)/2 = A000217(31).
		

Crossrefs

Cf. A000217, A068808.

Programs

Maple

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);

Formula

a(n) = (sqrt(1 + 8*A068808(n)) - 1)/2. - Jon E. Schoenfield, Dec 30 2023

Extensions

Better name and more terms from Jon E. Schoenfield, Dec 30 2023