0,2

Note that the empty set has even cardinality.

Seiichi Manyama, Table of n, a(n) for n = 0..210 (terms 0..80 from Alois P. Heinz)

a(n) = (2n)! * [x^(2n)] cosh(x)^(2n).

a(n) = Sum_{i=0..2*n} (n-i)^(2*n)*binomial(2*n,i). - Vladimir Kruchinin, Feb 07 2013

a(n) ~ c * n^(2*n) * 2^(2*n) * (1-r)^(2*n) / ((2-r)^n * r^n * exp(2*n)), where r = 0.1664434403990353015638385297757806508596082... is the root of the equation (2/r-1)^(1-r) = exp(2), and c = 1.66711311920192939687232294044843869828... = 2/A085984. - Vaclav Kotesovec, Sep 03 2014, updated Mar 18 2024

a(1) = 2 because there are 2 functions from {1,2} into {1,2} for which the preimage of both elements has even size: 1,1 (where the preimage of 1 is {1,2} and the preimage of 2 is the empty set) and 2,2 (where the preimage of 1 is the empty set and the preimage of 2 is {1,2}).

a:= n-> (2*n)! *coeff(series(cosh(x)^(2*n), x, 2*n+1), x, 2*n):

seq(a(n), n=0..20); # Alois P. Heinz, Jan 19 2013

nn=32; Select[Table[n!Coefficient[Series[Cosh[x]^n, {x, 0, nn}], x^n], {n, 0, nn}], #>0&]

a[ n_] := If[ n < 0, 0, With[{m = 2 n}, m! SeriesCoefficient[ Cosh[x]^m, {x, 0, m}]]]; (* Michael Somos, Jul 02 2017 *)

(PARI) {a(n) = if( n<0, 0, n=2*n; n! * polcoeff( cosh(x + x*O(x^n))^n, n))}; /* Michael Somos, Jul 02 2017 */

Cf. A085984.

Sequence in context: A000819 A060079 A052502 * A246742 A293950 A104134

Adjacent sequences: A209286 A209287 A209288 * A209290 A209291 A209292

Geoffrey Critzer, Jan 16 2013

approved