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 A001902 Successive denominators of Wallis's approximation to Pi/2 (reduced). 8
 1, 1, 3, 9, 45, 75, 175, 1225, 11025, 19845, 43659, 160083, 693693, 1288287, 2760615, 41409225, 703956825, 1329696225, 2807136475, 10667118605, 44801898141, 85530896451, 178837328943, 1371086188563, 11425718238025 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 REFERENCES H.-D. Ebbinghaus et al., Numbers, Springer, 1990, p. 146. LINKS J. Sondow, A faster product for Pi and a new integral for ln(Pi/2), arXiv:math/0401406 [math.NT], 2004. J. Sondow, A faster product for Pi and a new integral for ln(Pi/2), Amer. Math. Monthly 112 (2005), 729-734 and 113 (2006), 670. FORMULA (2*2*4*4*6*6*8*8*...*2n*2n*...)/(1*3*3*5*5*7*7*9*...*(2n-1)*(2n+1)*...) for n >= 1. From Wolfdieter Lang, Dec 07 2017: (Start) 1/1 * 2/1 * 2/3 * 4/3 * 4/5 * 6/5 * 6/7 * ...; partial products (reduced). Here the denominators with offset 0. a(n) = denominator(W(n)), for n >= 0, with W(n) = Product_{k=0..n} N(k)/D(k) (reduced), with N(k) = 2*floor((k+1)/2) for k >= 1 and N(0) = 1, and D(k) = 2*floor(k/2) + 1, for k >= 0. (End) EXAMPLE From Wolfdieter Lang, Dec 07 2017: (Start) See the table in A001901 for details. n = 5: numerator(1*2*2*4*4*6/(1*1*3*3*5*5)) = denominator(384/225) = denominator(128/75) = 75. (End) MATHEMATICA a[n_?EvenQ] := n!!^2/((n - 1)!!^2*(n + 1)); a[n_?OddQ] := (n - 1)!!^2*(n + 1)/n!!^2; Table[a[n] // Denominator, {n, 0, 23}] (* Jean-François Alcover, Jun 19 2013 *) CROSSREFS Numerators are A001901. For the unreduced form see A001900(n)/A000246(n+1), n >= 0. Sequence in context: A021103 A222662 A209977 * A224085 A192891 A068100 Adjacent sequences:  A001899 A001900 A001901 * A001903 A001904 A001905 KEYWORD nonn,frac,easy AUTHOR STATUS approved

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Last modified November 30 21:20 EST 2021. Contains 349426 sequences. (Running on oeis4.)