OFFSET
1,2
COMMENTS
1, 5286126, 45025305, 54742416, ... are also hexagonal and their sum of divisors too. - Michel Marcus, Apr 20 2014
LINKS
Donovan Johnson, Table of n, a(n) for n = 1..200
Shyam Sunder Gupta, Fascinating Triangular Numbers.
Douglas E. Iannucci, Florian Luca, Triangular numbers whose sum of divisors is also triangular, Acta Arithmetica 129(2007), 23-40.
EXAMPLE
a(2)=36 because sum of divisors of 36 =1+2+3+4+6+9+12+18+36=91, which is also a triangular number.
MATHEMATICA
Select[Accumulate[Range[120000]], OddQ[Sqrt[8*DivisorSigma[1, #]+1]]&] (* Harvey P. Dale, Feb 25 2015 *)
PROG
(PARI) isok(n) = ispolygonal(n, 3) && ispolygonal(sigma(n), 3); \\ Michel Marcus, Apr 20 2014
CROSSREFS
KEYWORD
nonn
AUTHOR
Shyam Sunder Gupta, Jun 15 2003
STATUS
approved