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

1,2

Comments

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

Links

Table of n, a(n) for n=1..42.

Formula

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

Example

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

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);

Crossrefs

Cf. A000217, A068808.

Sequence in context: A110449 A229082 A176363 * A114600 A088905 A045319

Adjacent sequences: A067988 A067989 A067990 * A067992 A067993 A067994

Keyword

nonn

Author

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

Extensions

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

Status

approved