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A239365 Numbers n such that 10*n^2+4 is a square. 2

%I

%S 12,456,17316,657552,24969660,948189528,36006232404,1367288641824,

%T 51920962156908,1971629273320680,74869991424028932,

%U 2843088044839778736,107962475712487563036,4099730989029687616632,155681815107415641868980,5911809243092764703404608

%N Numbers n such that 10*n^2+4 is a square.

%C Values of y satisfying the Pellian equation x^2 - 10*y^2 = 4.

%H Colin Barker, <a href="/A239365/b239365.txt">Table of n, a(n) for n = 1..600</a>

%H <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (38,-1).

%F a(n) = 2*A078987(n-1).

%F a(n) = 38*a(n-1)-a(n-2).

%F G.f.: 12*x / (x^2-38*x+1).

%e 456 is in the sequence because 10*456^2+4 = 2079364 = 1442^2.

%o (PARI) Vec(12/(x^2-38*x+1) + O(x^100))

%Y Cf. A078987, A239364.

%K nonn,easy

%O 1,1

%A _Colin Barker_, Mar 17 2014

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Last modified October 21 19:16 EDT 2021. Contains 348155 sequences. (Running on oeis4.)