

A309009


Numbers that are both triangular and octahedral.


1




OFFSET

1,3


COMMENTS

The list is probably finite, like the list of triangular and tetrahedral numbers (A027568).
It would appear that the similar sequence of pentagonaldodecahedral numbers contains only the trivial cases 0 and 1.
Intersection of A000217 and A005900.  A.H.M. Smeets, Aug 19 2019


LINKS

Table of n, a(n) for n=1..4.


MATHEMATICA

triQ[n_] := IntegerQ[Sqrt[8n + 1]]; oct[n_] := n(2n^2 + 1)/3 ; Select[oct@Range[0, 10], triQ] (* Amiram Eldar, Jul 15 2019 *)


PROG

(PARI) lista(nn) = for (n=0, nn, if (ispolygonal(k=n*(2*n^2 + 1)/3, 3), print1(k, ", "))); \\ Michel Marcus, Jul 06 2019


CROSSREFS

Cf. A000217, A005900, A027568, A307174.
Sequence in context: A282736 A277293 A177043 * A117064 A112001 A286314
Adjacent sequences: A309006 A309007 A309008 * A309010 A309011 A309012


KEYWORD

nonn,fini,more


AUTHOR

William C. Laursen, Jul 05 2019


STATUS

approved



