Abstract.
Let D be a digraph and let f be an invariant of D.
Then D is called
-
minimal by f, if f(D-e) differs from f(D) for
every arc e of D;
-
critical by f, if f(D-u) differs from f(D) for
every vertex v of D;
-
maximal by f, if f(D+e) differs from f(D) for
every arc e of the complement of D;
We consider digraphs minimal, critical and maximal by three types of
radii.
Some of the classes are completely characterized, while for the others we
show that they are large in terms of induced subgraphs.