OFFSET
1,2
LINKS
Donovan Johnson, Table of n, a(n) for n = 1..200
EXAMPLE
n=215, sigma(215)+sigma(216) = 264+600 = 2*432 = 2*sigma(215+216) = 2*sigma(431).
MATHEMATICA
Do[s=(DivisorSigma[1, n]+DivisorSigma[1, n+1])/ DivisorSigma[1, 2*n+1]; If[IntegerQ[s], Print[n]], {n, 1, 10000000}]
Position[Table[(DivisorSigma[1, n]+DivisorSigma[1, n+1])/DivisorSigma[ 1, 2n+1], {n, 28*10^5}], _?IntegerQ]//Flatten (* Harvey P. Dale, Nov 24 2017 *)
PROG
(PARI) is(n)=(sigma(n)+sigma(n+1))%sigma(2*n+1)==0 \\ Charles R Greathouse IV, Feb 14 2013
CROSSREFS
KEYWORD
nonn
AUTHOR
Labos Elemer, Feb 16 2004
STATUS
approved