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A279886 a(n) = A057863(n+1) * Sum_{k=0..n}(k! / (2*k+1)!!). 1
1, 4, 66, 7200, 6917400, 72442188000, 9822893856930000, 19942863749656848000000, 687732249521129504041350000000, 450437284567157389148103391935000000000, 6194243041031315772374678081343893262937500000000 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

LINKS

Daniel Suteu, Table of n, a(n) for n = 0..29

FORMULA

a(n) ~ Pi/2 * A057863(n+1).

a(0) = 1, a(n) = a(n-1) * (2*n+1)!! + n! * A057863(n).

MATHEMATICA

Table[Product[(2 k - 1)!!, {k, n + 1}] Sum[(j!/(2 j + 1)!!), {j, 0, n}], {n, 0, 10}] (* Michael De Vlieger, Dec 22 2016 *)

PROG

(PARI) a(n) = prod(k=0, n, prod(i=0, k, 2*i+1))*sum(k=0, n, k!/prod(i=0, k, 2*i+1));

CROSSREFS

Cf. A057863.

Sequence in context: A025585 A302657 A198893 * A048828 A225940 A003360

Adjacent sequences:  A279883 A279884 A279885 * A279887 A279888 A279889

KEYWORD

nonn,easy

AUTHOR

Daniel Suteu, Dec 21 2016

STATUS

approved

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Last modified May 29 10:05 EDT 2020. Contains 334699 sequences. (Running on oeis4.)