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A237420 If n is odd, then a(n) = 0; otherwise, a(n) = n. 3
0, 0, 2, 0, 4, 0, 6, 0, 8, 0, 10, 0, 12, 0, 14, 0, 16, 0, 18, 0, 20, 0, 22, 0, 24, 0, 26, 0, 28, 0, 30, 0, 32, 0, 34, 0, 36, 0, 38, 0, 40, 0, 42, 0, 44, 0, 46, 0, 48, 0, 50, 0, 52, 0, 54, 0, 56, 0, 58, 0, 60, 0, 62, 0, 64, 0, 66, 0, 68, 0, 70, 0, 72, 0, 74 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

Normally the OEIS excludes sequences in which every other term is zero. But there are exceptions for especially important sequences like this one. - N. J. A. Sloane, Feb 27 2014

Essentially the factorial expansion of exp(-1): exp(-1) = sum(n>=1, a(n)/(n+1)! ). [Joerg Arndt, Mar 13 2014]

a(n) is the number of m < n for which a(m) has the same parity as n. For instance, a(4) = 4 because 4 has the same parity as a(0), a(1), a(2), and a(3). - Alec Jones, May 16 2016

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 0..1000 (corrected by Ray Chandler, Jan 19 2019)

Index entries for linear recurrences with constant coefficients, signature (0,2,0,-1).

FORMULA

O.g.f.: 2*x^2/(1-x^2)^2.

E.g.f.: x*sinh(x). - Robert Israel, Aug 27 2015

a(n) = 2*a(n-2) - a(n-4) for n>4.

a(n) = 2*A142150(n) = (1+(-1)^n)*n/2 = n*((n-1) mod 2).

a(n) = floor(n^(-1)^n) for n>1. - Ilya Gutkovskiy, Aug 27 2015

Sum_{i=1..n} a(i) = A110660(n). - Bruno Berselli, Feb 27 2014

a(n) = -1 + ceiling((n + 1)^(sin(Pi*n/2) + cos(Pi*n))). - Lechoslaw Ratajczak, Nov 06 2016

MAPLE

seq(op([0, 2*i]), i=1..30); # Robert Israel, Aug 27 2015

MATHEMATICA

Table[If[OddQ[n], 0, n], {n, 80}]

CoefficientList[Series[2 x /(1 - x^2)^2, {x, 0, 80}], x]

LinearRecurrence[{0, 2, 0, -1}, {0, 0, 2, 0}, 75] (* Robert G. Wilson v, Nov 11 2016 *)

PROG

(MAGMA) [IsOdd(n) select 0 else n: n in [1..80]];

(MAGMA) [(1+(-1)^n)*n/2: n in [1..80]];

(MAGMA) &cat [[n, 0]: n in [0..80 by 2]]; // Bruno Berselli, Nov 11 2016

(PARI) a(n)=if(n%2==0, n, 0) \\ Anders Hellström, Aug 27 2015

CROSSREFS

Cf. A005843, A110660, A142150, A193356.

Sequence in context: A319813 A071648 A001613 * A130891 A091371 A144775

Adjacent sequences:  A237417 A237418 A237419 * A237421 A237422 A237423

KEYWORD

nonn,easy

AUTHOR

Vincenzo Librandi, Feb 24 2014

EXTENSIONS

Edited by Bruno Berselli, Feb 27 2014

STATUS

approved

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Last modified January 16 06:48 EST 2021. Contains 340204 sequences. (Running on oeis4.)