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 A046655 Sum of the squares of divisors is also a square number. 6

%I

%S 1,42,246,287,728,1434,1673,1880,4264,6237,9799,9855,18330,21352,

%T 21385,24856,36531,39990,46655,57270,66815,92664,125255,156570,182665,

%U 208182,212949,242879,273265,380511,391345,411558,539560,627215,693160,730145,741096

%N Sum of the squares of divisors is also a square number.

%D J.-M. De Koninck, Ces nombres qui nous fascinent, Entry 42, p. 16, Ellipses, Paris 2008.

%H M. F. Hasler and Donovan Johnson, <a href="/A046655/b046655.txt">Table of n, a(n) for n = 1..500</a> (first 99 terms from _M. F. Hasler_)

%H Project Euler, <a href="http://projecteuler.net/index.php?section=problems&amp;id=211">Problem 211: Divisor Square Sum</a>.

%e 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.

%t Select[Range[750000], IntegerQ[Sqrt[DivisorSigma[2, #]]] &] (* _Jayanta Basu_, Jun 27 2013 *)

%o (PARI) for( n=1,10^6, issquare(sigma(n,2)) && print1(n", ")) \\ _M. F. Hasler_, Oct 05 2008

%Y Cf. A046656, A000290, A001157.

%K nonn

%O 1,2

%A _Labos Elemer_

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Last modified August 18 01:19 EDT 2019. Contains 326059 sequences. (Running on oeis4.)