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A333320
a(n) is the number of subsets of {1..n} that contain exactly 4 odd and 1 even numbers.
2
0, 0, 0, 0, 0, 0, 0, 3, 4, 20, 25, 75, 90, 210, 245, 490, 560, 1008, 1134, 1890, 2100, 3300, 3630, 5445, 5940, 8580, 9295, 13013, 14014, 19110, 20475, 27300, 29120, 38080, 40460, 52020, 55080, 69768, 73644, 92055, 96900, 119700, 125685, 153615, 160930, 194810, 203665
OFFSET
0,8
COMMENTS
The general formula for the number of subsets of {1..n} that contain exactly k odd and j even numbers is binomial(ceiling(n/2), k) * binomial(floor(n/2), j).
LINKS
FORMULA
a(n) = binomial(ceiling(n/2),4) * floor(n/2).
From Colin Barker, Mar 17 2020: (Start)
G.f.: x^7*(3 + x + x^2) / ((1 - x)^6*(1 + x)^5).
a(n) = a(n-1) + 5*a(n-2) - 5*a(n-3) - 10*a(n-4) + 10*a(n-5) + 10*a(n-6) - 10*a(n-7) - 5*a(n-8) + 5*a(n-9) + a(n-10) - a(n-11) for n>10.
(End)
EXAMPLE
a(8)=4 and the 4 subsets are {1,2,3,5,7}, {1,3,4,5,7}, {1,3,5,6,7}, {1,3,5,7,8}.
MATHEMATICA
Array[Binomial[Ceiling[#], 4] Binomial[Floor[#], 1] &[#/2] &, 47, 0] (* Michael De Vlieger, Mar 14 2020 *)
PROG
(PARI) concat([0, 0, 0, 0, 0, 0, 0], Vec(x^7*(3 + x + x^2) / ((1 - x)^6*(1 + x)^5) + O(x^50))) \\ Colin Barker, Mar 17 2020
CROSSREFS
Cf. A333321.
Sequence in context: A051719 A336619 A240970 * A047165 A124631 A262033
KEYWORD
nonn,easy
AUTHOR
Enrique Navarrete, Mar 14 2020
STATUS
approved