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D-finite with recurrence (n+2)*a(n) -2*(n+1)*a(n-1) +4*(-2*n+3)*a(n-2)=0. - R. J. Mathar, Dec 10 2013
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3, 4, 9, 24, 70, 216, 693, 2288, 7722, 26520, 92378, 325584, 1158924, 4160240, 15043725, 54747360, 200360130, 736928280, 2722540590, 10098646800, 37594507860, 140415097680, 526024740930, 1976023374624, 7441754696100, 28091245875056, 106268257060308, 402815053582368
From Amiram Eldar, Feb 14 2023: (Start)
Sum_{n>=0} 1/a(n) = Pi/(9*sqrt(3)) + 5/9.
Sum_{n>=0} (-1)^n/a(n) = 17/75 - 22*log(phi)/(75*sqrt(5)), where phi is the golden ratio (A001622). (End)
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(MAGMAMagma) [6*Binomial(2*n, n)/(n+2): n in [0..30]]; // Vincenzo Librandi, May 10 2012
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