OFFSET
1,2
COMMENTS
Subsequence of A116990. - Michel Marcus, Jun 13 2015
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..100
EXAMPLE
11 is in the sequence because sigma(11*12/2) = sigma(66) = 144 = 12^2 = sigma(11)^2.
MATHEMATICA
Select[Range@1000000, DivisorSigma[1, #]^2==DivisorSigma[1, (# (# + 1)/2)] &] (* Vincenzo Librandi, Jun 13 2015 *)
PROG
(PARI) isok(n) = sigma(n)^2 == sigma(n*(n+1)/2); \\ Michel Marcus, Nov 23 2013
(Magma) [n: n in [1..7*10^5] | SumOfDivisors(n*(n+1) div 2) eq SumOfDivisors(n)^2]; // Vincenzo Librandi, Jun 13 2015
CROSSREFS
Cf. A116990 (indices of triangular numbers whose sum of divisors is square).
KEYWORD
nonn,changed
AUTHOR
Alex Ratushnyak, Nov 22 2013
EXTENSIONS
More terms from Michel Marcus, Nov 23 2013
STATUS
approved