Claspers and finite type invariants of links

We introduce the concept of "claspers," which are surfaces in 3-manifolds with some additional structure on which surgery operations can be performed. Using claspers we de fi ne for each positive integer k an equivalence relation on links called "C-k-equivalence," which is generated by surgery operations of a certain kind called "C-k-moves". We prove that two knots in the 3-sphere are Ck+ 1-equivalent if and only if they have equal values of Vassiliev-Goussarov invariants of type k with values in any abelian groups. This result gives a characterization in terms of surgery operations of the informations that can be carried by Vassiliev-Goussarov invariants. In the last section we also describe outlines of some applications of claspers to other fi elds in 3-dimensional topology.