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A187274 a(n) = n*4^(n/2-1)*(9+(-1)^n). 4
0, 4, 20, 48, 160, 320, 960, 1792, 5120, 9216, 25600, 45056, 122880, 212992, 573440, 983040, 2621440, 4456448, 11796480, 19922944, 52428800, 88080384, 230686720, 385875968, 1006632960, 1677721600, 4362076160, 7247757312, 18790481920, 31138512896, 80530636800, 133143986176, 343597383680 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

LINKS

Table of n, a(n) for n=0..32.

R. Kemp, On the number of words in the language {w in Sigma* | w = w^R }^2, Discrete Math., 40 (1982), 225-234. See Lemma 1.

Index entries for linear recurrences with constant coefficients, signature (0,8,0,-16).

FORMULA

a(n) = 8*a(n-2) - 16*a(n-4). - Colin Barker, Jul 25 2013

G.f.: 4*x*(x+1)*(4*x+1) / ((2*x-1)^2*(2*x+1)^2). - Colin Barker, Jul 25 2013

a(2*n) = 5*n*4^n, a(2*n+1) = (2*n+1)*4^(n+1). - Andrew Howroyd, Mar 28 2016

MAPLE

See A187272.

MATHEMATICA

LinearRecurrence[{0, 8, 0, -16}, {0, 4, 20, 48}, 40] (* Harvey P. Dale, Dec 25 2014 *)

PROG

(MAGMA) /* By definition: */ [Integers()!(n*4^(n/2-1)*(9+(-1)^n)): n in [0..40]]; // Bruno Berselli, Mar 29 2016

(MAGMA) I:=[0, 4, 20, 48]; [n le 4 select I[n] else 8*Self(n-2)-16*Self(n-4): n in [1..30]]; // Vincenzo Librandi, Mar 29 2016

CROSSREFS

Sequence in context: A033579 A294630 A160799 * A108099 A244050 A250224

Adjacent sequences:  A187271 A187272 A187273 * A187275 A187276 A187277

KEYWORD

nonn,easy

AUTHOR

N. J. A. Sloane, Mar 07 2011

STATUS

approved

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Last modified May 23 03:18 EDT 2018. Contains 304447 sequences. (Running on oeis4.)