OFFSET
1,2
COMMENTS
The corresponding triangular indices are in A116990. - Michel Marcus, Mar 17 2015
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
EXAMPLE
3 is in the sequence because 3=2*3/2 is triangular, and sigma(3)=1+3=4=2^2 is square.
MATHEMATICA
Select[Accumulate[Range[0, 2000]], IntegerQ@Sqrt@DivisorSigma[1, #] &] (* Michael De Vlieger, Mar 17 2015 *)
PROG
(PARI) {for(i=1, 2*10^3, n=i*(i+1)/2; if(issquare(sigma(n)), print1(n, ", ")))}
(Magma) [n*(n+1) div 2: n in [1..2000] | IsSquare(SumOfDivisors(n*(n+1) div 2))]; // Vincenzo Librandi, Mar 17 2015
CROSSREFS
KEYWORD
nonn
AUTHOR
Antonio Roldán, Mar 16 2015
STATUS
approved