A necessary and sufficient condition for a graph $G$, which satisfies   the equality $\mu_{21}(G)=|V(G)|$
  A necessary and sufficient condition is found for a graph $G$, which satisfies the equality $\mu_{21}(G)=|V(G)|$. 
Narine N. Davtyan, Rafayel R. Kamalian
cs.DM