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 A239435 Values of n for which the equation x^2 - 25*y^2 = n has integer solutions. 2

%I

%S 0,1,4,9,11,16,21,24,25,31,36,39,41,44,49,51,56,61,64,69,71,75,81,84,

%T 91,96,99,100,101,104,111,119,121,124,125,129,131,136,141,144,151,156,

%U 159,161,164,169,171,175,176,181,184,189,191,196,200,201,204,211

%N Values of n for which the equation x^2 - 25*y^2 = n has integer solutions.

%C This equation is a Pellian equation of the form x^2 - D^2*y^2 = N.

%H Colin Barker, <a href="/A239435/b239435.txt">Table of n, a(n) for n = 1..1000</a>

%e 21 is in the sequence because the equation x^2 - 25*y^2 = 21 has the solution (X,Y) = (11,2).

%o (PARI)

%o b(n) = sumdiv(n, f, f^2<=n && (n-f^2)%(10*f)==0) \\ See A239434

%o s=[]; for(n=0, 250, if(b(n)>0, s=concat(s,n))); s

%Y Cf. A042965, A230239, A230240, A239434.

%K nonn

%O 1,3

%A _Colin Barker_, Mar 18 2014

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Last modified October 20 20:04 EDT 2019. Contains 328270 sequences. (Running on oeis4.)