OFFSET
0,2
LINKS
James Mitchell, Table of n, a(n) for n = 0..100
B. M. Schein and B. Teclezghi, Endomorphisms of finite symmetric inverse semigroups, J. Algebra, 198 (1997) 300-310.
PROG
(PARI) a(n)=if(n==0, return (1), if(n<5, return([3, 14, 54, 306][n]))); n!*(3+sum(m=0, n, sum(k=0, m\2, 2^(m-3*k)/((n-m)!*(m-2*k)!*k!))))
CROSSREFS
KEYWORD
nonn
AUTHOR
James Mitchell, Jun 08 2013
STATUS
approved