OFFSET
1,4
COMMENTS
Consider vertices numbered 1 to n. Add the edges 1--n, 2--n, and either 1--(n+1-k), 2--(n+2-k), ... or k--n for 3<=k<n. (Altogether (n-1)!/2 possibilities.) If the resulting graph has no odd cycles, this is a graceful labeling of a bipartite graph.
EXAMPLE
For example, the seven labelings for n=5 are:
1--5, 2--5, 1--3, 2--3;
1--5, 2--5, 1--3, 3--4;
1--5, 2--5, 1--3, 4--5;
1--5, 2--5, 2--4, 2--3;
1--5, 2--5, 2--4, 3--4;
1--5, 2--5, 3--5, 3--4;
1--5, 2--5, 3--5, 4--5.
The first of these is the four-cycle plus an isolated vertex; the other six are the trees in A337274.
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Don Knuth, Sep 08 2020
EXTENSIONS
a(17)-a(18) from Bert Dobbelaere, Sep 11 2020
STATUS
approved