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A130318 Integer values of k!!/S(k), where S(k) is the sum of all odd numbers less than or equal to k, if k is odd, or the sum of all even numbers less than or equal to k, if k is even.

%I A130318 #25 Jun 22 2025 03:08:39
%S A130318 1,1,4,128,11520,143360,425425,2064384,619315200,280284364800,
%T A130318 6801567252480,27512370460575,178541140377600,152355106455552000,
%U A130318 167834385271436083200,6074006324109115392000,29734853645550994565625,231916605102348042240000,392866729043377583554560000
%N A130318 Integer values of k!!/S(k), where S(k) is the sum of all odd numbers less than or equal to k, if k is odd, or the sum of all even numbers less than or equal to k, if k is even.
%C A130318 For n >= 8, a(n) ends with 0 or 5.
%H A130318 Jayanta Basu, <a href="/A130318/b130318.txt">Table of n, a(n) for n = 0..100</a>
%F A130318 Integers of the form k!!/((k+1)/2)^2, for k odd and k!!/(k*(k+2)/4) for k even. [corrected by _Jon E. Schoenfield_, Mar 16 2024]
%e A130318 6 --> 6!! = 48; 6 + 4 + 2 = 12; 48/12 = 4.
%e A130318 17 --> 17!! = 34459425; 17 + 15 + 13 + 11 + 9 + 7 + 5 + 3 + 1 = 81; 34459425/81 = 425425.
%p A130318 f:= n-> `if`(irem(doublefactorial(n), floor((n+1)^2/4), 'r')=0, r, [][]):
%p A130318 map(f, [$1..50])[];  # _Alois P. Heinz_, Mar 16 2024
%t A130318 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 *)
%Y A130318 Cf. A108552, A000290, A005408, A130319.
%Y A130318 Cf. A002620, A006882.
%K A130318 easy,nonn
%O A130318 0,3
%A A130318 _Paolo P. Lava_ and _Giorgio Balzarotti_, May 23 2007