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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
define for each positive integer k an equivalence relation
on links called "Ck–equivalence," which
is generated by surgery operations of a certain kind called
"Ck–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 fields in
3–dimensional topology.
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