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A061981 a(n) = 3^n - 2n - 1. 5
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 and 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)

From Deisy J. Camacho, Feb 26 2021 (Start)

a(n) = Sum_{j=2..n} Sum_{i=0..j} n!/((j-i)!*i!*(n-j)!).

a(n) = 4 + 4*a(n-1) - 3*a(n-2). (End)

MATHEMATICA

Table[3^n-2n-1, {n, 0, 30}] (* or *) LinearRecurrence[{5, -7, 3}, {0, 0, 4}, 30] (* Harvey P. Dale, Mar 30 2018 *)

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: A291526 A303011 A197426 * A054611 A329174 A196432

Adjacent sequences:  A061978 A061979 A061980 * A061982 A061983 A061984

KEYWORD

nonn,easy

AUTHOR

Henry Bottomley, May 24 2001

STATUS

approved

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Last modified April 22 14:39 EDT 2021. Contains 343177 sequences. (Running on oeis4.)