OFFSET
1,2
COMMENTS
It is not known if this sequence is infinite.
a(15) > 10^11. - Donovan Johnson, Mar 17 2011
The standard heuristic suggests this sequence is infinite with logarithmic density. - Charles R Greathouse IV, Feb 04 2013
FORMULA
n such that sigma(1)+sigma(2)+...+sigma(n) = x^2 with some integer x.
EXAMPLE
a(2) = 2 since sigma(1) = 1^2 and sigma(1)+sigma(2) = 2^2.
MATHEMATICA
L=0; For[j = 1, j < 1000000, j++, L = L + DivisorSigma[1, j]; If[L == Floor[Sqrt[L]]^2, Print[j, " ", Floor[Sqrt[L]]]]]
PROG
(PARI) s=0; for(n=1, 1e6, if(issquare(s+=sigma(n)), print1(n", "))) \\ Charles R Greathouse IV, Feb 04 2013
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Florian Luca (fluca(AT)matmor.unam.mx), Jul 11 2007
EXTENSIONS
a(10)-a(13) from Donovan Johnson, Dec 02 2009
a(14) from Donovan Johnson, Mar 17 2011
Name revised by Charles R Greathouse IV, Feb 04 2013
a(15)-a(20) from Giovanni Resta, Nov 22 2019
STATUS
approved