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A001901
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Successive numerators of Wallis's approximation to Pi/2 (reduced).
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5
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1, 2, 4, 16, 64, 128, 256, 2048, 16384, 32768, 65536, 262144, 1048576, 2097152, 4194304, 67108864, 1073741824, 2147483648, 4294967296, 17179869184, 68719476736, 137438953472, 274877906944, 2199023255552
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OFFSET
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0,2
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REFERENCES
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H.-D. Ebbinghaus et al., Numbers, Springer, 1990, p. 146.
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LINKS
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Eric Weisstein's World of Mathematics, Pi
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FORMULA
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(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.
1/1 * 2/1 * 2/3 * 4/3 * 4/5 * 6/5 * 6/7 * ...; partial products (reduced). Here the numerators with offset 0.
a(n) = numerator(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)
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EXAMPLE
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The Wallis numerators (N) and denominators (D) with partial products A(n) = A001900(n) and B(n) = A000246(n+1) in unreduced form, and a(n) and b(n) = A001902(n) in reduced form.
n, k: 0 1 2 3 4 5 6 7 8 9 10 ...
N(k): 1 2 2 4 4 6 6 8 8 10 10 ...
D(k): 1 1 3 3 5 5 7 7 9 9 9 ...
A(n): 1 2 4 16 64 384 2304 18432 147456 1474560 14745600 ...
B(n): 1 1 3 9 45 225 1575 11025 99225 893025 9823275 ...
a(n): 1 2 4 16 64 128 256 2048 16384 32768 65536 ...
b(n): 1 1 3 9 45 75 175 1225 11025 19845 43659 ...
n = 5: numerator(1*2*2*4*4*6/(1*1*3*3*5*5)) = numerator(384/225) = numerator(128/75) = 128. (End)
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MATHEMATICA
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a[n_?EvenQ] := n!!^2/((n - 1)!!^2*(n + 1)); a[n_?OddQ] := ((n - 1)!!^2*(n + 1))/n!!^2; Table[a[n] // Numerator, {n, 0, 23}] (* Jean-François Alcover, Jun 19 2013 *)
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CROSSREFS
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KEYWORD
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nonn,frac,easy
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AUTHOR
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STATUS
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approved
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