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A025303 Numbers that are the sum of 2 distinct nonzero squares in exactly 2 ways. 4

%I

%S 65,85,125,130,145,170,185,205,221,250,260,265,290,305,340,365,370,

%T 377,410,442,445,481,485,493,500,505,520,530,533,545,565,580,585,610,

%U 625,629,680,685,689,697,730,740,745,754,765,785,793,820,865,884,890,901,905,949

%N Numbers that are the sum of 2 distinct nonzero squares in exactly 2 ways.

%C Numbers with exactly 2 distinct prime divisors of the form 4k+1, each with multiplicity one, or with only one prime divisor of this form with multiplicity 3, and with no prime divisor of the form 4k+3 to an odd multiplicity. - _Jean-Christophe Hervé_, Dec 01 2013

%H Donovan Johnson, <a href="/A025303/b025303.txt">Table of n, a(n) for n = 1..1000</a>

%H <a href="/index/Su#ssq">Index entries for sequences related to sums of squares</a>

%F A004018(a(n)) = 16. - _Jean-Christophe Hervé_, Dec 01 2013

%e 65 = 5*13 = 64+1 = 49 + 16; 85 = 5*17 = 81+4 = 49+16; 125 = 5^3 = 121+4 = 100+25; 130 = 2*5*13 = 121+9 = 81+49. - _Jean-Christophe Hervé_, Dec 01 2013

%t nn = 949; t = Table[0, {nn}]; lim = Floor[Sqrt[nn - 1]]; Do[num = i^2 + j^2; If[num <= nn, t[[num]]++], {i, lim}, {j, i - 1}]; Flatten[Position[t, 2]] (* _T. D. Noe_, Apr 07 2011 *)

%Y Cf. A001481, A004431, A004018, A230779 (one way).

%Y Cf. analogs for square decompositions: A084645, A084646, A084647, A084648, A084649.

%K nonn

%O 1,1

%A _David W. Wilson_

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Last modified November 21 04:47 EST 2019. Contains 329350 sequences. (Running on oeis4.)