OFFSET
1,2
COMMENTS
From Zak Seidov, Oct 19 2010: (Start)
Corresponding values of s begin: 1,2,12,24,42,72,96,216,240,192,240,288,336,456,504, 480,600,672,840,720,720,792,960,930,936,756,992,936,1008,1080,... (are most values of s multiples of 3?).
(End)
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000 (terms 1..561 from Zak Seidov)
FORMULA
EXAMPLE
a(1) = 1 because sigma(1*2/2) = sigma(1) = 1 = 1^2,
a(2) = 2 because sigma(2*3/2) = sigma(3) = 2^2,
a(3) = 11 because sigma(11*12/2) = sigma(66) = 144 = 12^2.
MAPLE
with(numtheory): a:=proc(n) if type(sqrt(sigma(n*(n+1)/2)), integer)=true then n else fi end: seq(a(n), n=0..3100); # Emeric Deutsch, Apr 06 2006
MATHEMATICA
Flatten@ Position[Accumulate[Range@ 2600], n_ /; IntegerQ@ Sqrt@ DivisorSigma[1, n] == True] (* Michael De Vlieger, Mar 17 2015 *)
Select[Range[2600], IntegerQ[Sqrt[DivisorSigma[1, (#(#+1))/2]]]&] (* Harvey P. Dale, Nov 19 2022 *)
PROG
(PARI) for(n=1, 1000, if(issquare(sigma(n*(n+1)/2)), print1(n", "))) \\ Zak Seidov, Mar 21 2015
CROSSREFS
KEYWORD
easy,nonn,less
AUTHOR
Jonathan Vos Post, Apr 04 2006
EXTENSIONS
More terms from Emeric Deutsch, Apr 06 2006
Incorrect term 0 removed by Michel Marcus, Mar 17 2015
STATUS
approved