  Bound muons (also pions, kaons, etc) increase the fission barrier and produce
some stabilizing effects for highly charged nuclei. If the binding energy of
the muon exceeds $mc^2$, it becomes stable. The $1s$ state of a muon inside an
exotic nucleus with atomic number $A= 5Z/2$ and such large charge $Z$ that the
$1s$ energy $E$ is in the range $0\le E<-mc^2$ is considered.
