OFFSET
1,2
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
EXAMPLE
a(1) = 1 because sigma(1*(3*1-1)/2) = 1 = 1^2.
a(2) = 4 because sigma(4*(3*4-1)/2) = 36 = 6^2.
a(3) = 7 because sigma(7*(3*7-1)/2) = 144 = 12^2.
a(4) = 12 because sigma(12*(3*12-1)/2) = 576 = 24^2.
a(5) = 21 because sigma(21*(3*21-1)/2) = 1024 = 32^2.
a(6) = 23 because sigma(23*(3*23-1)/2) = 1296 = 36^2.
a(7) = 27 because sigma(27*(3*27-1)/2) = 3600 = 60^2.
a(8) = 31 because sigma(31*(3*31-1)/2) = 2304 = 48^2.
a(9) = 71 because sigma(71*(3*71-1)/2) = 11664 = 108^2.
MAPLE
with(numtheory): select(n-> sqrt(sigma(n*(3*n-1)/2))::integer, [$1..2200])[]; # Emeric Deutsch, Apr 06 2006
MATHEMATICA
s = {}; Do[If[IntegerQ @ Sqrt @ DivisorSigma[1, (3 n - 1)*n/2], AppendTo[s, n]], {n, 1, 2000}]; s (* Amiram Eldar, Aug 17 2019 *)
Position[DivisorSigma[1, PolygonalNumber[5, Range[2000]]], _?(IntegerQ[ Sqrt[ #]]&)]//Flatten (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Oct 23 2020 *)
PROG
(PARI) isok(n) = issquare(sigma(n*(3*n-1)/2)); \\ Michel Marcus, Aug 17 2019
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Jonathan Vos Post, Apr 04 2006
EXTENSIONS
More terms from Emeric Deutsch, Apr 06 2006
a(0) removed by Amiram Eldar, Aug 17 2019
STATUS
approved