Maciej Borodzik: Seifert matrices and the unknotting number
We shall define a new unknotting invariant n(K) for a knot K in S^3,
which can be computed from a Seifert matrix of a knot.
We can show that it is --- up to now -- the best unknotting invariant
that can be computed from a Seifert matrix only. We show some evidence
that n(K) might be in fact equal to the algebraic unknotting. As an
application we show a simple and easy-to-use obstruction for the
knot to have unknotting number 2. This is joint work with Stefan Friedl