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 A061981 a(n) = 3^n - 2n - 1. 4
 0, 0, 4, 20, 72, 232, 716, 2172, 6544, 19664, 59028, 177124, 531416, 1594296, 4782940, 14348876, 43046688, 129140128, 387420452, 1162261428, 3486784360, 10460353160, 31381059564, 94143178780, 282429536432, 847288609392 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 LINKS Harry J. Smith, Table of n, a(n) for n = 0..200 Brandon Alberts, Jack Klys, The distribution of H8-extensions of quadratic fields, arXiv:1611.05595 [math.NT], 2016. See p. 17. Index entries for linear recurrences with constant coefficients, signature (5,-7,3). FORMULA From Bruno Berselli, Jan 31 2012: (Start) G.f.: 4*x^2/((1-3*x)*(1-x)^2). a(n) = A186948(n)-1. a(n+2) = 4*A000340(n). (End) PROG (PARI) { for (n=0, 200, write("b061981.txt", n, " ", 3^n - 2*n - 1) ) } \\ Harry J. Smith, Jul 29 2009 CROSSREFS Column of A061980. Sequence in context: A123613 A006740 A197426 * A054611 A196432 A196508 Adjacent sequences:  A061978 A061979 A061980 * A061982 A061983 A061984 KEYWORD nonn,easy AUTHOR Henry Bottomley, May 24 2001 STATUS approved

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