Completions of 2-torsion KN-algebras of genus 1

Krichever-Novikov algebras ICN of genus 1 with markings which are two 2-tor sion points are related to a family W-f of deformations of the Witt algebra W, where f varies in the space of even polynomials with vanishing constant terms. An isomorphism between the formal (resp. analytic) completion of th ese KN-algebras with those of the Witt algebra is proved. Central extension s of these algebras are also defined and their formal completion is proved to be isomorphic to that of the Virasoro algebra Vir.