cp's OEIS Frontend

A131574 Numbers n that are the product of two distinct odd primes and x^2 + y^2 = n has integer solutions.

%I A131574 #30 Dec 09 2019 12:57:58
%S A131574 65,85,145,185,205,221,265,305,365,377,445,481,485,493,505,533,545,
%T A131574 565,629,685,689,697,745,785,793,865,901,905,949,965,985,1037,1073,
%U A131574 1145,1157,1165,1189,1205,1241,1261,1285,1313,1345,1385,1405,1417,1465,1469
%N A131574 Numbers n that are the product of two distinct odd primes and x^2 + y^2 = n has integer solutions.
%C A131574 The two primes are of the form 4*k + 1.
%H A131574 Ray Chandler, <a href="/A131574/b131574.txt">Table of n, a(n) for n = 1..10000</a> (first 1000 terms from Colin Barker)
%e A131574 65 is in the sequence because x^2 + y^2 = 65 = 5*13 has solutions (x,y) = (1,8), (4,7), (7,4) and (8,1).
%o A131574 (PARI)
%o A131574 dop(d, nmax) = {
%o A131574   my(L=List(), v=vector(d,m,1)~, f);
%o A131574   for(n=1, nmax,
%o A131574     f=factorint(n);
%o A131574     if(#f~==d && f[1,1]>2 && f[,2]==v && f[,1]%4==v, listput(L, n))
%o A131574   );
%o A131574   Vec(L)
%o A131574 }
%o A131574 dop(2, 3000) \\ _Colin Barker_, Nov 15 2015
%Y A131574 Cf. A000415, A121387, A248649, A248712, A264498, A264499
%K A131574 nonn
%O A131574 1,1
%A A131574 _Colin Barker_, Aug 28 2007, corrected Aug 29 2007