A225505 a(n) = triangular(a(n-1)+a(n-2)) if n > 1, else a(n) = n.

0, 1, 1, 3, 10, 91, 5151, 13741903, 94490753712985, 4464252567106907867941273716, 9964775491460730298984873904585383048580645394630925051

Offset

0,4

Links

Table of n, a(n) for n=0..10.

Mathematica

nxt[{a_, b_}]:=With[{c=a+b}, {b, (c(c+1))/2}]; Join[{0}, NestList[nxt, {1, 1}, 10][[;; , 1]]] (* Harvey P. Dale, Jan 16 2026 *)

Prog

(Python)
prpr, prev = 0, 1
for i in range(1, 17):
  print(prpr, end=', ')
  n = prpr+prev
  cur = n*(n+1)//2
  prpr, prev = prev, cur

Crossrefs

Cf. A000217, A072638.

Sequence in context: A224774 A034792 A135457 * A073733 A005205 A216450

Adjacent sequences: A225502 A225503 A225504 * A225506 A225507 A225508

Keyword

nonn

Author

Alex Ratushnyak, May 09 2013

Status

approved