OFFSET
1,1
COMMENTS
Numbers n such that sum of the squares of the proper (or aliquot) divisors of n is a prime number.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
FORMULA
EXAMPLE
a(3)=21 because the aliquot divisors of 21 are 1, 3, 7, the sum of whose squares is 1^2 + 3^2 + 7^2 = 59, prime.
MAPLE
A067558 := proc(n)
numtheory[sigma][2](n)-n^2 ;
end proc:
isA201880 := proc(n)
isprime(A067558(n)) ;
end proc:
for n from 1 to 1000 do
if isA201880(n) then
printf("%d, ", n);
end if;
end do: # R. J. Mathar, Dec 07 2011
MATHEMATICA
Select[Range[400], PrimeQ[DivisorSigma[2, #]-#^2]&]
PROG
(PARI) is(n)=isprime(sigma(n, 2)-n^2) \\ Charles R Greathouse IV, Dec 06 2011
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Lagneau, Dec 06 2011
STATUS
approved