

A152665


Number of leading even entries in all permutations of {1,2,...,n}.


2



0, 1, 2, 16, 60, 540, 3024, 32256, 241920, 3024000, 28512000, 410572800, 4670265600, 76281004800, 1017080064000, 18598035456000, 284549942476800, 5762136335155200, 99527809425408000, 2211729098342400000, 42575785143091200000, 1030334000462807040000
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OFFSET

1,3


LINKS



FORMULA

a(n) = Sum_{k=0..floor(n/2)} k*A152664(n,k).
a(2n+1) = n(2n+1)!/(n+2);
a(2n) = n(2n)!/(n+1).
Dfinite with recurrence 2*(n+3)*a(n) +(5*n8)*a(n1) +(2*n^32*n^2n4)*a(n2) +(n2)*(3*n^23*n+2)*a(n3) +(n3)*(n2)^2*a(n4)=0.  R. J. Mathar, Jul 26 2022


EXAMPLE

The permutation 4,6,2,1,5,3 begins with three even numbers, so would contribute 3 to a(6).
a(3)=2 because in the permutations 123, 132, 213, 231, 312, 321 we have 0+0+1+1+0+0 = 2 leading odd entries.
a(45) = 16: Here are the permutations of 1234, 24 in all:
1(234) total 6, no. of initial even terms = 0
3(124) ditto
21(34) total 2, no. of initial even terms 1*2 = 2
23(14) ditto
24(13) total 2, no. of initial even terms 2 twice = 4
Subtotal from 2*** is 2+2+4 = 8
Subtotal from 4*** is also 2+2+4 = 8
Total a(4) = 8+8 = 16.


MAPLE

ao := proc (n) options operator, arrow; n*factorial(2*n+1)/(n+2) end proc: ae := proc (n) options operator, arrow; n*factorial(2*n)/(n+1) end proc: a := proc (n) if `mod`(n, 2) = 1 then ao((1/2)*n1/2) else ae((1/2)*n) end if end proc; seq(a(n), n = 1 .. 20);


MATHEMATICA

a[n_] := If[OddQ[n], (n1)*n!/(n+3), n*n!/(n+2)];


CROSSREFS



KEYWORD

nonn


AUTHOR



EXTENSIONS



STATUS

approved



