A072521 a(1) = 6 and then the smallest triangular numbers such that sum of two neighbors is also a triangular number.

6, 15, 21, 45, 91, 990, 1711, 365085, 401856, 713415, 785631, 1079715, 1326006, 2355535, 2888406, 5137615, 5666661, 5764710, 9550635, 9921285, 10934826, 19434495, 21421785, 23622501, 42003195, 46315500, 82349361, 146384605

Offset

1,1

Comments

The sequence is unbounded as a(n+1) is less than or equal to the n-th triangular number.

Links

Table of n, a(n) for n=1..28.

Example

45 is a term because 21 + 45 = 66, 45 + 91 = 136, and 66 and 136 are triangular numbers.

Prog

(PARI) p=6; k=3; print1(p", "); for(n=1, 30, k=k+1; u=p+k*(k+1)/2; t=floor(sqrt(2*u)); while(u!=t*(t+1)/2, k=k+1; u=p+k*(k+1)/2; t=floor(sqrt(2*u))); p=k*(k+1)/2; print1(p", "))

Crossrefs

Cf. A072522.

Sequence in context: A332877 A357325 A362211 * A130178 A100410 A095032

Adjacent sequences: A072518 A072519 A072520 * A072522 A072523 A072524

Keyword

nonn

Author

Amarnath Murthy, Jul 31 2002

Extensions

More terms from Ralf Stephan, Mar 30 2003

Status

approved