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A009123 E.g.f.: cosh(log(1+sin(x))). 2

%I

%S 1,0,1,-3,8,-30,136,-693,3968,-25260,176896,-1351383,11184128,

%T -99680490,951878656,-9695756073,104932671488,-1202439837720,

%U 14544442556416,-185185594118763,2475749026562048,-34674437196568950

%N E.g.f.: cosh(log(1+sin(x))).

%C |a(n)| = number of even alternating permutations on n letters (offset 1). - _Vladeta Jovovic_, May 20 2007

%F a(2n) = 1/2*A000182(n+1), a(2n+1) = A012007(n+1) = A009567(2n+1)+1.

%F G.f.: (1+x/(1+x^2))/2 + 1/2/Q(0) where Q(k) = 1 + (k+1)*x - x^2*(k+1)*(k+2)/2 /Q(k+1) ; (continued fraction). - _Sergei N. Gladkovskii_, Mar 12 2013

%F a(n) ~ n! * n * (-1)^n * (2/Pi)^(n+2). - _Vaclav Kotesovec_, Jan 22 2015

%t CoefficientList[Series[(1 + (1 + Sin[x])^2)/(2*(1 + Sin[x])), {x, 0, 20}], x] * Range[0, 20]! (* _Vaclav Kotesovec_, Jan 22 2015 *)

%Y Cf. A000111.

%K sign,easy

%O 0,4

%A _R. H. Hardin_

%E Extended with signs by _Olivier GĂ©rard_, Mar 15 1997

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Last modified February 21 19:50 EST 2018. Contains 299423 sequences. (Running on oeis4.)