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 A000819 E.g.f.: cos(x)^2 / cos(2x) = Sum_{n >= 0} a(n) * x^(2n) / (2n)!. 2
 1, 2, 40, 1952, 177280, 25866752, 5535262720, 1633165156352, 635421069967360, 315212388819402752, 194181169538675507200, 145435130631317935357952, 130145345400688287667978240, 137139396592145493713802493952 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 LINKS R. J. Mathar, Table of n, a(n) for n = 1..200 FORMULA E.g.f.: cos(x)^2/cos(2x)=1/Q(0)+1/2;   Q(k)=1+1/(1-2*(x^2)/(2*(x^2)-(k+1)*(2k+1)/Q(k+1)));  (continued fraction). - Sergei N. Gladkovskii, Nov 18 2011 From Michael Somos, Apr 04 2017: (Start) E.g.f.: cos(x)^2 / cos(2*x) = (1 + sec(2*x)) / 2 = tan(2*x) / (2 * tan(x)) = 1 / (1 - tan(x)^2). a(n) = A000816(n) unless n=0. a(n) = 1/2 * A002436(n) unless n=0. a(n) = 2^(2*n - 1) * A000364(n). (End) EXAMPLE G.f. = 1 + 2*x + 40*x^2 + 1952*x^3 + 177280*x^4 + 25866752*x^5 + ... - Michael Somos, Apr 04 2017 MATHEMATICA With[{nn=30}, Take[CoefficientList[Series[Cos[x]^2/Cos[2x], {x, 0, nn}], x] Range[ 0, nn]!, {1, -1, 2}]] (* Harvey P. Dale, Jul 06 2014 *) a[ n_] := If[ n < 0, 0, With[ {m = 2 n}, m! SeriesCoefficient[ 1 / (1 - Tan[x]^2), {x, 0, m}]]]; (* Michael Somos, Apr 04 2017 *) PROG (PARI) {a(n) = my(m); if( n<0, 0, m = 2*n; m! * polcoeff( 1 / (1 - tan(x + x * O(x^m))^2), m))}; /* Michael Somos, Apr 04 2017 */ CROSSREFS Essentially the same as A000816. Second column of array A103905. Cf. A000364, A002436. Sequence in context: A261732 A163826 A000816 * A060079 A052502 A209289 Adjacent sequences:  A000816 A000817 A000818 * A000820 A000821 A000822 KEYWORD nonn AUTHOR STATUS approved

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