Incidences between points and curves ($I(P,\mathcal{C})=|\{(p,\gamma ):p\in \gamma \}|$)

For finite sets of points $P$ and geometric curves $\mathcal{C}$, a point-curve incidence is a pair $(p,\gamma )$ with $p\in \gamma$. The incidence count is $I(P,\mathcal{C})={\sum }_{\gamma \in \mathcal{C}}|P\cap \gamma |$. Bounds in incidence geometry exploit restrictions on how curves can meet points, rather than treating the incidence relation as an arbitrary bipartite graph.

If $n$ points and $m$ curves have the property that each pair of distinct points lies on at most $\lambda$ curves, then double counting gives ${\sum }_{\gamma }{k}_{\gamma }(k_{\gamma }-1)\le \lambda n(n-1)$, where $k_{\gamma }=|P\cap \gamma |$. The Cauchy-Schwarz inequality implies $I^2\le mI+\lambda mn(n-1)$ and hence $I\le m+\sqrt{\lambda mn(n-1)}$. The statement needs only this combinatorial multiplicity condition, not algebraicity.