OFFSET
1,2
REFERENCES
D. E. Knuth, The Art of Computer Programming, Vol. 4A, Section 7.1.4.
LINKS
Andrew Howroyd, Table of n, a(n) for n = 1..50
PROG
(PARI)
GraphsWithDegreeAtMost(n, limit)={
local(M=Map());
my(acc(p, v)=my(z); mapput(M, p, if(mapisdefined(M, p, &z), z+v, v)));
my(recurse(p, i, q, v, e)=if(e<=limit&&poldegree(q)<=limit, if(i<0, acc(x^e+q, v), my(t=polcoeff(p, i)); for(k=0, t, self()(p, i-1, (t-k+x*k)*x^i+q, binomial(t, k)*v, e+k)))));
my(iterate(v, k, f)=for(i=1, k, v=f(v)); v);
vecsum(Vec(iterate(Mat([1, 1]), n-1, src->M=Map(); for(i=1, matsize(src)[1], my(p=src[i, 1]); recurse(p, poldegree(p), 0, src[i, 2], 0)); Mat(M)))[2]); }
a(n) = GraphsWithDegreeAtMost(n, 4); \\ Andrew Howroyd, Aug 25 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Don Knuth, Mar 31 2008
EXTENSIONS
Terms a(13) and beyond from Andrew Howroyd, Aug 25 2017
STATUS
approved