OFFSET
2,1
REFERENCES
R. C. Read, Some Enumeration Problems in Graph Theory. Ph.D. Dissertation, Department of Mathematics, Univ. London, 1958.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
LINKS
Andrew Howroyd, Table of n, a(n) for n = 2..50
Sean A. Irvine, Illustration of initial terms
R. C. Read, Letter to N. J. A. Sloane, Feb 04 1971 (gives initial terms of this sequence)
PROG
(PARI)
dpermcount(v) = {my(m=1, s=0, k=0, t); for(i=1, #v, t=v[i]; k=if(i>1&&t==v[i-1], k+1, 1); m*=2*t*k; s+=2*t); s!/m}
S(n, x)={vector(n, n, if(n>1, sum(k=0, n, binomial(2*n-k, k)*2*n/(2*n-k)*x^k), 0))}
q(n, s)={my(t=0); if(n>1, forpart(p=n, t+=dpermcount(p)*prod(i=1, #p, s[p[i]]), [2, n])); t}
a(n)={my(p=q(n, S(n, x))); sum(i=0, poldegree(p), polcoeff(p, n-i)*(-1)^(n-i)*(2*i)!/(2^i*i!))} \\ Andrew Howroyd, Dec 18 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
EXTENSIONS
a(5)-a(6) corrected and a(7)-a(10) from Sean A. Irvine, Oct 05 2014
Terms a(11) and beyond from Andrew Howroyd, Dec 18 2017
STATUS
approved