A Remark on Classical Pluecker's formulae

Abstract

For any reduced curve $C\subset \mathbb P^2$, we define the notions of the number of its virtual cusps $c_v$ and the number of its virtual nodes $n_v$ which are non-negative, coincide respectively with the numbers of ordinary cusps and nodes in the case of cuspidal curves, and if $\hat C$ is the dual curve of an irreducible curve $C$ and $\hat n_v$ and $\hat c_v$ are the numbers of its virtual nodes and virtual cusps, then the integers $c_v$, $n_v$, $\hat c_v$, $\hat n_v$ satisfy Classical Pl\"{u}cker's formulae.