Monotonic invariants under blowups

We prove that the numerical invariant $3\mu-4\tau$ of a reduced irreducible plane curve singularity germ is non-negative, non-decreasing under blowups and strictly increasing unless the curve is non-singular. This provides a new perspective to understand the question posed by A. Dimca and G.-M. Greuel. Moreover, our work can be put in the general framework of discovering monotonic invariants under blowups.