Differential algebras of Legendrian links

The problem of classification of Legendrian knots (links) up to isotopy in the class of Legendrian embeddings (Legendrian isotopy) naturally leads to the following two subproblems. The first of them is: which combinations of the three classical invariants can be realized by a Legendrian knot? (It is well-known that each invariant by itself can be realized by a Legendrian knot). The first step in this direction was done by Bennequin who proved that the Bennequin number of a Legendrian knot is less than twice its genus. The second subproblem is whether there exist a pair of Legendrian knots which have the same classical invariants but are not Legendrian isotopic. Eliashberg and Fraser showed that this is impossible when knots are trivial as smooth knots. In the present paper, we develop a theory which allows us, in particular, to find a counterexample to this subproblem.

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