OFFSET
1,2
COMMENTS
If n is prime, the sum is equal to 1.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
FORMULA
EXAMPLE
The divisors of 465 are {1, 3, 5, 15, 31, 93, 155, 465} and the sum of the nonprime divisors 1 + 15 + 93 + 155 + 465 = 729 = 27^2, hence 465 is in the sequence.
MAPLE
A023890 := proc(n) a := 0 ; for d in numtheory[divisors](n) do if not isprime(d) then a := a+d; end if; end do; a; end proc:
end do: # R. J. Mathar, Sep 06 2011
MATHEMATICA
f[n_] := IntegerQ[Sqrt[Total[Select[Divisors[n], ! PrimeQ[#] &]]]]; Select[Range[25553], ! PrimeQ[#] && f[#] &] (* T. D. Noe, Sep 06 2011 *)
PROG
(PARI) isok(n) = !isprime(n) && issquare(sumdiv(n, d, d*(1-isprime(d)))); \\ Michel Marcus, Aug 25 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Lagneau, Aug 29 2011
STATUS
approved