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 A212303 a(n) = n!/([(n-1)/2]!*[(n+1)/2]!) for n>0, a(0)=0, and where [ ] = floor. 5
 0, 1, 2, 3, 12, 10, 60, 35, 280, 126, 1260, 462, 5544, 1716, 24024, 6435, 102960, 24310, 437580, 92378, 1847560, 352716, 7759752, 1352078, 32449872, 5200300, 135207800, 20058300, 561632400, 77558760, 2326762800, 300540195, 9617286240, 1166803110, 39671305740 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS a(n) + A056040(n) = A189911(n), the row sums of the extended Catalan triangle A189231. LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..1000 Peter Luschny, The lost Catalan numbers. FORMULA E.g.f.: (1+x)*BesselI(1, 2*x). O.g.f.: -((4*x^2-1)^(3/2)+I-(4*I)*x^2+(4*I)*x^3)/(2*x*(4*x^2-1)^(3/2)). Recurrence: a(n) = n if n < 2 else a(n) = a(n-1)*n if n is even else a(n-1)*n*4/((n-1)*(n+1)). a(2*n) = n*C(2*n, n) (A005430); a(2*n+1) = C(2*n+1,  n+1) (A001700). a(n) = n\$*floor((n+1)/2)^((-1)^n), where n\$ is the swinging factorial A056040. a(n) = sum_{k=0..n} A189231(n, 2*k+1). sum_{n>=1} 1/a(n) = 2/3 + (7/27)*sqrt(3)*Pi. MAPLE A212303 := proc(n) if n mod 2 = 0 then n*binomial(n, iquo(n, 2))/2 else binomial(n+1, iquo(n, 2)+1)/2 fi end: seq(A212303(i), i=0..36); MATHEMATICA a[n_?EvenQ] := n*Binomial[n, n/2]/2; a[n_?OddQ] := Binomial[n+1, Quotient[n, 2]+1]/2; Table[a[n], {n, 0, 36}]  (* Jean-François Alcover, Feb 05 2014 *) nxt[{n_, a_}]:={n+1, If[OddQ[n], a(n+1), (4a(n+1))/(n(n+2))]}; Join[{0}, Transpose[ NestList[ nxt, {1, 1}, 40]][[2]]] (* Harvey P. Dale, Dec 20 2014 *) PROG (Sage) def A212303():     yield 0     r, n = 1, 1     while True:         yield r         n += 1         r *= n if is_even(n) else 4*n/((n-1)*(n+1)) a = A212303(); [a.next() for i in range(36)] CROSSREFS Cf. A005430, A001700, A056040, A189911. Sequence in context: A168059 A068550 A093432 * A100561 A081529 A002944 Adjacent sequences:  A212300 A212301 A212302 * A212304 A212305 A212306 KEYWORD nonn AUTHOR Peter Luschny, Oct 24 2013 STATUS approved

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Last modified October 13 18:14 EDT 2019. Contains 327981 sequences. (Running on oeis4.)