multiplicity of a curve on a surface This comes as a question in Beauville's surfaces book (Chap2.20.(2)).
Let $C$ be an irreducible curve on a surface $S$, $p\in C$ and $C'$ the strict transform of $C$ on the blow-up with center $p$.
The question is:

*

*$\sum m_x(C')\le\sum m_x(C' \cap E)$, where the sum runs through $x \in C'$ lying over $p$ ($E$: exceptional curve).


*find an example with strict inequality.
I can't do them. Also I don't know I understand them.
In case that $C$ :cusp and $p\in C$:singular point , can it be answer of 2)?
Any idea or help for 1) & 2)?
 A: Let us assume, as Beauville does in his book, that the surface $S$ is smooth.  The exceptional divisor $E$ is thus isomorphic to $\mathbb P^1$, a smooth divisor on the smooth surface $\hat{S}$.  The inequality actually holds point by point, so fix $x\in C'\cap E$.  Since multiplicities are a local invariant, we work in the completed local ring of $\hat{S}$ at $x$, which is isomorphic to $\mathbb C[[s,y]]$.  We may even choose coordinates so that $E$ is locally cut out by $(y)$, and $C'$ by $f\in (s,y)\subset \mathbb C[s,y]$.  Since $C'$ is the strict transform, it shares no component with $E$, so $f\notin (y)$.
By $m_x(C'\cap E)$, he means the length of the scheme-theoretic intersection, which is a 0-dimensional scheme.  If $C'$ is locally cut out by $f\in \mathbb C[[s,y]]$, then
$$m_x(C'\cap E) = \dim \mathbb C[[s,y]]/(y,f)$$
which is the minimal degree of a monomial in $f$ not divisible by $y$.  By $m_x(C')$, he means the degree of $f\in \mathbb C[s,y]$, that is the minimal degree of a monomial in $f$. Thus,
$$m_x(C')\leq m_x(C'\cap E).$$
For the example, you are correct in taking $C$ to be a cuspidal curve, say $V(y^2-t^3)\subset \mathbb A^2_{(t,y)}$.  In the blow up, $C'$ is cut out by $s^2-y$ on the appropriate affine patch of $\hat{S}$.  Here, $m_x(C'\cap E)=2$ whereas $m_x(C')=1$.
