prop.gd: add "condition true → property false" implications

digraphs · Oct 27, 2021 · 507c008 · 507c008
1 parent ce01c04
commit 507c008
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Showing 3 changed files with 40 additions and 6 deletions.
diff --git a/gap/prop.gd b/gap/prop.gd
@@ -106,3 +106,41 @@ InstallTrueMethod(DigraphHasEdges, IsDigraph and DigraphHasLoops);
 InstallTrueMethod(DigraphHasLoops, IsReflexiveDigraph and DigraphHasVertices);
 InstallTrueMethod(DigraphHasVertices, IsDigraph and DigraphHasEdges);
 InstallTrueMethod(DigraphHasVertices, IsDigraph and IsDirectedTree);
+
+# Implications that something is false
+InstallTrueMethod(HasDigraphHasLoops, IsAcyclicDigraph);
+InstallTrueMethod(HasDigraphHasLoops, IsTournament);
+InstallTrueMethod(HasDigraphHasLoops, IsUndirectedForest);
+InstallTrueMethod(HasDigraphHasLoops, IsDirectedTree);
+InstallTrueMethod(HasDigraphHasLoops, IsEmptyDigraph);
+InstallTrueMethod(HasDigraphHasLoops, IsCompleteDigraph and DigraphHasEdges);
+InstallTrueMethod(HasDigraphHasLoops, IsBipartiteDigraph);
+
+# The following are valid, but unnecessary because of immediate methods
+InstallTrueMethod(HasDigraphHasEdges, IsEmptyDigraph);
+InstallTrueMethod(HasIsEmptyDigraph, IsDigraph and DigraphHasEdges);
+
+InstallTrueMethod(HasIsAcyclicDigraph, IsCompleteDigraph and DigraphHasEdges);
+InstallTrueMethod(HasIsAcyclicDigraph, IsDigraph and DigraphHasLoops);
+InstallTrueMethod(HasIsAcyclicDigraph,
+                  IsStronglyConnectedDigraph and DigraphHasEdges);
+InstallTrueMethod(HasIsAntisymmetricDigraph,
+                  IsCompleteDigraph and DigraphHasEdges);
+InstallTrueMethod(HasIsChainDigraph, IsDigraph and DigraphHasLoops);
+InstallTrueMethod(HasIsChainDigraph, IsSymmetricDigraph and DigraphHasEdges);
+InstallTrueMethod(HasIsCompleteDigraph, IsDigraph and DigraphHasLoops);
+InstallTrueMethod(HasIsReflexiveDigraph,
+                  IsAcyclicDigraph and DigraphHasVertices);
+
+InstallTrueMethod(HasIsSymmetricDigraph, IsAcyclicDigraph and DigraphHasEdges);
+InstallTrueMethod(HasIsSymmetricDigraph, IsDirectedTree and DigraphHasEdges);
+InstallTrueMethod(HasIsSymmetricDigraph, IsTournament and DigraphHasEdges);
+
+InstallTrueMethod(HasIsMultiDigraph, IsChainDigraph);
+InstallTrueMethod(HasIsMultiDigraph, IsCompleteDigraph);
+InstallTrueMethod(HasIsMultiDigraph, IsCompleteMultipartiteDigraph);
+InstallTrueMethod(HasIsMultiDigraph, IsCycleDigraph);
+InstallTrueMethod(HasIsMultiDigraph, IsEmptyDigraph);
+InstallTrueMethod(HasIsMultiDigraph, IsFunctionalDigraph);
+InstallTrueMethod(HasIsMultiDigraph, IsTournament);
+InstallTrueMethod(HasIsMultiDigraph, IsUndirectedForest);
diff --git a/gap/prop.gi b/gap/prop.gi
@@ -200,10 +200,6 @@ function(D)
   return DigraphNrConnectedComponents(D) = 1;
 end);
 
-InstallImmediateMethod(IsAcyclicDigraph, "for a reflexive digraph",
-IsReflexiveDigraph, 0,
-D -> DigraphNrVertices(D) = 0);
-
 InstallImmediateMethod(IsAcyclicDigraph, "for a strongly connected digraph",
 IsStronglyConnectedDigraph, 0,
 D -> DigraphNrVertices(D) <= 1 and IsEmptyDigraph(D));

diff --git a/tst/standard/prop.tst b/tst/standard/prop.tst
@@ -1,7 +1,7 @@
 #############################################################################
 ##
 #W  standard/prop.tst
-#Y  Copyright (C) 2014-17                                James D. Mitchell
+#Y  Copyright (C) 2014-21                                James D. Mitchell
 ##                                                          Wilf A. Wilson
 ##
 ##  Licensing information can be found in the README file of this package.
@@ -139,7 +139,7 @@ gap> gr := Digraph([[1]]);;
 gap> DigraphHasLoops(gr);
 true
 gap> HasIsAcyclicDigraph(gr);
-false
+true
 gap> IsAcyclicDigraph(gr);
 false
 gap> gr := Digraph([[2], []]);