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trying to enhance graph products #2576

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trying to enhance graph products #2576

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3d2c19c
Update Graphs.m2
fchapoton Aug 22, 2022
54a4ed1
Update Graphs.m2
fchapoton Aug 22, 2022
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28 changes: 20 additions & 8 deletions M2/Macaulay2/packages/Graphs.m2
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Original file line number Diff line number Diff line change
Expand Up @@ -1360,17 +1360,17 @@ isWeaklyConnected Digraph := Boolean => D -> isConnected underlyingGraph D

cartesianProduct = method()
cartesianProduct(Graph, Graph) := Graph => (G, H) -> (
V := toList(set vertexSet G ** set vertexSet H);
V := vertexSet G ** vertexSet H;
E := flatten for u in V list for v in V list
if (u_0 == v_0 and member(set {u_1, v_1}, edges H))
or (u_1 == v_1 and member(set {u_0, v_0}, edges G))
if (u_0 === v_0 and member(set {u_1, v_1}, edges H))
or (u_1 === v_1 and member(set {u_0, v_0}, edges G))
then {u, v} else continue;
graph(V, E, EntryMode => "edges")
)

-- the 'directProduct' method is defined in 'Polyhedra'
directProduct(Graph,Graph) := Graph => (G, H) -> (
V := toList(set vertexSet G ** set vertexSet H);
V := vertexSet G ** vertexSet H;
E := flatten for u in V list for v in V list
if member(set {u_0, v_0}, edges G) and member(set {u_1, v_1}, edges H)
then {u, v} else continue;
Expand All @@ -1387,10 +1387,10 @@ disjointUnion List := Graph => L -> (

graphComposition = method()
graphComposition (Graph, Graph) := Graph => (G, H) -> (
V := toList(set vertexSet G ** set vertexSet H);
V := vertexSet G ** vertexSet H;
E := flatten for u in V list for v in V list
if member(set {u_0, v_0}, edges G)
or (u_0 == v_0 and member(set {u_1, v_1}, edges H))
or (u_0 === v_0 and member(set {u_1, v_1}, edges H))
then {u, v} else continue;
graph(V, E, EntryMode => "edges")
)
Expand All @@ -1410,8 +1410,8 @@ strongProduct = method()
strongProduct (Graph, Graph) := Graph => (G, H) -> (
V := toList \ toList(set vertexSet G ** set vertexSet H);
E' := flatten for u in V list for v in V list
if (u_0 == v_0 and member(set {u_1, v_1}, edges H))
or (u_1 == v_1 and member(set {u_0, v_0}, edges G))
if (u_0 === v_0 and member(set {u_1, v_1}, edges H))
or (u_1 === v_1 and member(set {u_0, v_0}, edges G))
then {u, v} else continue;
E'' := flatten for u in V list for v in V list
if member(set {u_0, v_0}, edges G) and member(set {u_1, v_1}, edges H)
Expand Down Expand Up @@ -5509,6 +5509,18 @@ TEST ///
assert Equation(degreeSequence pathGraph 5, {2, 2, 2, 1, 1})
///

TEST ///
--check products with arbitrary labels
G = graph{{a,1}};
H = graph{{b,2}};
cp = cartesianProduct(G,H);
assert #(vertexSet cp) == 4
dp = directProduct(G,H);
assert #(vertexSet dp) == 4
gc = graphComposition(G,H);
assert #(vertexSet gc) == 4
///

end;

loadPackage(Graphs, Reload => true)
Expand Down