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 A158317 400n - 1. 2
 399, 799, 1199, 1599, 1999, 2399, 2799, 3199, 3599, 3999, 4399, 4799, 5199, 5599, 5999, 6399, 6799, 7199, 7599, 7999, 8399, 8799, 9199, 9599, 9999, 10399, 10799, 11199, 11599, 11999, 12399, 12799, 13199, 13599, 13999, 14399, 14799, 15199 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The identity (400*n-1)^2-(400*n^2-2*n)*(20)^2=1 can be written as a(n)^2-A158316(n)*(20)^2=1. LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..10000 Vincenzo Librandi, X^2-AY^2=1 E. J. Barbeau, Polynomial Excursions, Chapter 10: Diophantine equations (2010), pages 84-85 (row 15 in the first table at p. 85, case d(t) = t*(20^2*t-2)). Index entries for linear recurrences with constant coefficients, signature (2,-1). FORMULA a(n) = 2*a(n-1)-a(n-2). G.f.: x*(399+x)/(1-x)^2. MATHEMATICA LinearRecurrence[{2, -1}, {399, 799}, 50] PROG (MAGMA) I:=[399, 799]; [n le 2 select I[n] else 2*Self(n-1)-Self(n-2): n in [1..50]]; (PARI) a(n) = 400*n - 1. CROSSREFS Cf. A158316. Sequence in context: A176911 A202158 A126231 * A227008 A253597 A006972 Adjacent sequences:  A158314 A158315 A158316 * A158318 A158319 A158320 KEYWORD nonn,easy AUTHOR Vincenzo Librandi, Mar 16 2009 STATUS approved

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Last modified December 6 14:47 EST 2019. Contains 329806 sequences. (Running on oeis4.)