%I #37 Mar 19 2020 16:33:45
%S 1,1,8,26,352,1936,38528,297296,7869952,78098176,2583554048,
%T 31336418816,1243925143552,17831101321216,825787662368768,
%U 13658417358350336,722906928498737152,13551022195053101056
%N Expansion of e.g.f. (sin x + cos x)/cos 3x.
%H R. J. Mathar, <a href="/A000810/b000810.txt">Table of n, a(n) for n = 0..200</a>
%F a(2n) = A000436(n).
%F (-1)^n*a(2n+1)=1-sum_{i=0,1,...,n-1} (-1)^i*binomial(2n+1,2i+1)*3^(2n-2i)*a(2i+1). - _R. J. Mathar_, Nov 19 2006
%F a(n) = | 3^n*2^(n+1)*lerchphi(-1,-n,1/3) |. - _Peter Luschny_, Apr 27 2013
%F a(n) ~ n!*2^(n+1)*3^(n+1/2)/Pi^(n+1) if n is even and a(n) ~ n!*2^(n+1)*3^n/Pi^(n+1) if n is odd. - _Vaclav Kotesovec_, Jun 25 2013
%F a(n) = (-1)^floor(n/2)*3^n*skp(n, 1/3), where skp(n,x) are the Swiss-Knife polynomials A153641. - _Peter Luschny_, Apr 19 2014
%t CoefficientList[Series[(Sin[x]+Cos[x])/Cos[3*x], {x, 0, 20}], x]* Range[0, 20]! (* _Vaclav Kotesovec_, Jun 25 2013 *)
%t Table[Abs[EulerE[n, 1/3]] 6^n, {n, 0, 20}] (* _Vladimir Reshetnikov_, Oct 21 2015 *)
%o (Sage)
%o from mpmath import mp, lerchphi
%o mp.dps = 32; mp.pretty = True
%o def A000810(n): return abs(3^n*2^(n+1)*lerchphi(-1,-n,1/3))
%o [int(A000810(n)) for n in (0..17)] # _Peter Luschny_, Apr 27 2013
%o (PARI) x='x+O('x^66); v=Vec(serlaplace( (sin(x)+cos(x)) / cos(3*x) ) ) \\ _Joerg Arndt_, Apr 27 2013
%Y Cf. A007286, A007289.
%Y (-1)^(n*(n-1)/2)*a(n) gives the alternating row sums of A225118. - _Wolfdieter Lang_, Jul 12 2017
%K nonn
%O 0,3
%A _N. J. A. Sloane_