A046655 Numbers whose sum of the squares of divisors is also a square number.

1, 42, 246, 287, 728, 1434, 1673, 1880, 4264, 6237, 9799, 9855, 18330, 21352, 21385, 24856, 36531, 39990, 46655, 57270, 66815, 92664, 125255, 156570, 182665, 208182, 212949, 242879, 273265, 380511, 391345, 411558, 539560, 627215, 693160, 730145, 741096

Offset: 1

Author

Labos Elemer

Keywords

• nonn

Examples

			a(3) = 246 = 2*3*41 with 8 divisors: 1, 2, 3, 6, 41, 82, 123, 246. The sum of squares of the divisors is 84100 = 290^2, also a square.
		

References

• Jean-Marie De Koninck, Ces nombres qui nous fascinent, Entry 42, p. 16, Ellipses, Paris, 2008.

Links

• Amiram Eldar, Table of n, a(n) for n = 1..1000 (terms 1..99 from M. F. Hasler, terms 100..500 from Donovan Johnson)
• Project Euler, Problem 211: Divisor Square Sum.

Crossrefs

Cf. A046656, A000290, A001157.

Programs

Mathematica

Select[Range[750000], IntegerQ[Sqrt[DivisorSigma[2, #]]] &] (* Jayanta Basu, Jun 27 2013 *)

PARI

for( n=1,10^6, issquare(sigma(n,2)) && print1(n", ")) \\ M. F. Hasler, Oct 05 2008