OFFSET
1,1
COMMENTS
This property is shared with all the primes since sigma_2(p) = 1 + p^2.
The values of sqrt(sigma_2(a(n))-1) are 7, 47, 157, 3107, 3863, 54243, 257843, 6349657, 2207848187.
Are there terms not of the form 2^k * p where p is prime? - David A. Corneth, Aug 20 2018
2*10^12 < a(10) <= 44463118771144. The terms 21687324345660824, 14524130539077100050485512, 287674439504279743204606472 (and others) of the form 2^k * p can be found by solving the quadratic Diophantine equation sigma_2(2^k) * (p^2 + 1) = x^2 + 1 for appropriate values of k. - Giovanni Resta, Aug 20 2018
MATHEMATICA
sQ[n_] := IntegerQ[Sqrt[n]]; aQ[n_] := CompositeQ[n] && sQ[DivisorSigma[2, n]-1]; Select[Range[10000], aQ]
PROG
(PARI) forcomposite(n=2, 1e15, if( issquare(sigma(n, 2)-1), print1(n, ", ")))
(Magma) [n: n in [2..6*10^6] |not IsPrime(n) and IsSquare(DivisorSigma(2, n)-1)]; // Vincenzo Librandi, Aug 22 2018
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Amiram Eldar, Aug 20 2018
STATUS
approved