OFFSET
1,4
COMMENTS
A simple graph, also called a strict graph, is an unweighted, undirected graph containing no graph loops or multiple edges. Given an undirected graph, a degree sequence is a monotonic nonincreasing sequence of the vertex degrees (valencies) of its graph vertices.
LINKS
Darryn Bryant and Tom McCourt, Graphs with five vertices, Figure 1.
Eric W. Weisstein, MathWorld: Simple Graph
EXAMPLE
a(3) = 1 because there is a unique graph with 3 vertices each having prime degree, the triangle, with degree sequence (2,2,2).
a(4) = 3 because there are 3 graphs with 4 vertices each having prime degree: the 4-cycle (2,2,2,2); the complete graph K_4 with degree sequence (3,3,3,3); and two triangle graphs sharing a common edge, with degree sequence (3,3,2,2).
a(5) = 4 because there are 4 graphs with 5 vertices each having prime degree: the 5-cycle with degree sequence (2,2,2,2,2); a square graph sharing an edge with a triangle graph (G_13 in the linked-to illustration) with degree sequence (3,3,2,2,2); G_14 in the linked-to illustration with degree sequence (3,3,2,2,2); G_18 in the linked-to illustration with degree sequence (3,3,3,3,2).
MATHEMATICA
a[n_Integer] := Count[And @@ PrimeQ /@ GraphData[#, "Degrees"] & /@ GraphData[n], True] (* Charles R Greathouse IV, Apr 11 2012 *)
show[n_Integer] := Map[Graph, GraphData[#, "EdgeRules"] & /@
Select[GraphData[n], And @@ PrimeQ /@ GraphData[#, "Degrees"] &]] (* Charles R Greathouse IV, Apr 12 2012 *)
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Jonathan Vos Post, Apr 11 2012
EXTENSIONS
a(5)-a(7) from Charles R Greathouse IV, Apr 11 2012
a(8)-a(15) from Andrew Howroyd, Mar 08 2020
a(16)-a(20) from Andrew Howroyd, May 03 2020
STATUS
approved