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A130318 Integer values of n!!/S(n), where S(n) is the sum of all odd numbers minor or equal to n, if n is odd, or the sum of all even numbers minor or equal to n, if n is even. 3
1, 1, 4, 128, 11520, 143360, 425425, 2064384, 619315200, 280284364800, 6801567252480, 27512370460575, 178541140377600, 152355106455552000, 167834385271436083200, 6074006324109115392000, 29734853645550994565625 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

After the eighth term all the numbers end with 0 or 5.

LINKS

Jayanta Basu, Table of n, a(n) for n = 0..100

FORMULA

Integer of the form n!!/A005408, for n odd and n!!/A002378 (offset 2) for n even.

EXAMPLE

6 --> 6!! = 48; 6 + 4 + 2 = 12; 48/12 = 4.

17 --> 17!! = 34459425; 17 + 15 + 13 + 11 + 9 + 7 + 5 + 3 + 1 = 81; 34459425/81 = 425425.

MAPLE

P:=proc(n) local a, i, j, k, w; for i from 1 by 1 to n do k:=i; w:=i-2; while w>0 do k:=k*w; w:=w-2; od; j:=i; w:=i-2; while w>0 do j:=j+w; w:=w-2; od; a:=k/j; if trunc(a)=a then print(a) fi; od; end: P(100);

MATHEMATICA

Select[Table[Times @@ (t = If[OddQ[n], Range[1, n, 2], Range[2, n, 2]])/Plus @@ t, {n, 41}], IntegerQ] (* Jayanta Basu, Aug 12 2013 *)

CROSSREFS

Cf. A108552, A000290, A005408, A130319.

Sequence in context: A267796 A013823 A321233 * A000318 A229385 A141367

Adjacent sequences: A130315 A130316 A130317 * A130319 A130320 A130321

KEYWORD

easy,nonn

AUTHOR

Paolo P. Lava and Giorgio Balzarotti, May 23 2007

STATUS

approved

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Last modified December 4 02:16 EST 2022. Contains 358544 sequences. (Running on oeis4.)