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 A144965 a(n)= 4*n*(4*n^2+1). 2
 0, 20, 136, 444, 1040, 2020, 3480, 5516, 8224, 11700, 16040, 21340, 27696, 35204, 43960, 54060, 65600, 78676, 93384, 109820, 128080, 148260, 170456, 194764, 221280, 250100, 281320, 315036, 351344, 390340, 432120, 476780, 524416, 575124 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS (a(n))^2 + (n*a(n)+1)^2 is always a perfect square. LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..1000 Luc Comeau-Montasse, Des mesures entieres pour des triangles rectangles Index to sequences with linear recurrences with constant coefficients, signature (4,-6,4,-1). FORMULA G.f.: 4*x*(5+14*x+5*x^2)/(1-x)^4. [Colin Barker, May 24 2012] a(n) = 4*a(n-1) -6*a(n-2) +4*a(n-3) -a(n-4). - Vincenzo Librandi, Jun 30 2012 EXAMPLE Example : n = 181, a(n) = 94876580 (94876580^2 + (94876580*181 + 1)^2) = 17172923069. MATHEMATICA CoefficientList[Series[4*x*(5+14*x+5*x^2)/(1-x)^4, {x, 0, 40}], x] (* Vincenzo Librandi, Jun 30 2012 *) PROG (MAGMA) I:=[0, 20, 136, 444]; [n le 4 select I[n] else 4*Self(n-1)-6*Self(n-2)+4*Self(n-3)-Self(n-4): n in [1..50]]; // Vincenzo Librandi, Jun 30 2012 CROSSREFS Sequence in context: A168178 A085284 A105573 * A140301 A134382 A105939 Adjacent sequences:  A144962 A144963 A144964 * A144966 A144967 A144968 KEYWORD easy,nonn AUTHOR Luc Comeau-Montasse, Sep 27 2008 STATUS approved

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