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 A143382 Numerator of Sum_{k=0..n} 1/k!!. 2
 1, 2, 5, 17, 71, 121, 731, 1711, 41099, 370019, 740101, 2713789, 1206137, 423355111, 846710651, 1814380259, 203210595443, 12654139763, 531473870981, 43758015399281, 525096184837561, 441080795274037, 22054039763790029 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Denominators are A143383. A143382(n)/A143383(n) is to A007676(n)/A007676(n) as double factorials are to factorials. A143382/A143383 fractions begin: n numerator/denominator 0 1/0!! = 1/1 1 1/0!! + 1/1!! = 2/1 2 1/0!! + 1/1!! + 1/2!! = 5/2 3 1/0!! + 1/1!! + 1/2!! + 1/3!! = 17/6 4 1/0!! + 1/1!! + 1/2!! + 1/3!! + 1/4!! = 71/24 5 1/0!! + 1/1!! + 1/2!! + 1/3!! + 1/4!! + 1/5!! = 121/40 6 1/0!! + 1/1!! + 1/2!! + 1/3!! + 1/4!! + 1/5!! + 1/6!! = 731/240 The series converges to sqrt(e) + sqrt((e*Pi)/2)*erf(1/sqrt(2)) = 3.0594074053425761445... whose decimal expansion is given by A143280. The analogs of A094007 and A094008 are determined by 2 being the only prime denominator in the convergents to the sum of reciprocals of double factorials and prime numerators beginning: a(1) = 2, a(2) = 5, a(3) = 17, a(4) = 71, a(15) = 1814380259, a(19) = 43758015399281, a(21) = 441080795274037, a(23) = 867081905243923. LINKS G. C. Greubel, Table of n, a(n) for n = 0..500 Eric W. Weisstein, Double Factorial. Gives formula for limit of series, which was independently derived by Max Alekseyev. FORMULA Numerators of Sum_{k=0..n} 1/k!! = Sum_{k=0..n} 1/A006882(k). EXAMPLE a(3) = 17 because 1/0!! + 1/1!! + 1/2!! + 1/3!! = 17/6. a(15) = 1814380259 because 1814380259/593049600 = 1/1 + 1/1 + 1/2 + 1/3 + 1/8 + 1/15 + 1/48 + 1/105 + 1/384 + 1/945 + 1/3840 + 1/10395 + 1/46080 + 1/135135 + 1/645120 + 1/2027025. MATHEMATICA Table[Numerator[Sum[1/k!!, {k, 0, n}]], {n, 0, 30}] (* G. C. Greubel, Mar 28 2019 *) PROG (PARI) vector(25, n, n--; numerator(sum(k=0, n, 1/prod(j=0, floor((k-1)/2), (k - 2*j)) ))) \\ G. C. Greubel, Mar 28 2019 (MAGMA) [n le 0 select 1 else Numerator( 1 + (&+[ 1/(0 + (&*[k-2*j: j in [0..Floor((k-1)/2)]])) : k in [1..n]]) ): n in [0..25]]; // G. C. Greubel, Mar 28 2019 (Sage) [numerator(sum( 1/product((k - 2*j) for j in (0..floor((k-1)/2)))   for k in (0..n))) for n in (0..25)] # G. C. Greubel, Mar 28 2019 CROSSREFS Cf. A006882 (n!!), A094007, A143280 (m(2))), A143383 (denominators). Sequence in context: A317132 A335926 A139402 * A057219 A084869 A101900 Adjacent sequences:  A143379 A143380 A143381 * A143383 A143384 A143385 KEYWORD easy,frac,nonn AUTHOR Jonathan Vos Post, Aug 11 2008 STATUS approved

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Last modified November 27 16:35 EST 2021. Contains 349394 sequences. (Running on oeis4.)