  In his pioneering work from 1969, Jerry Levine introduced a complete set of
invariants of algebraic concordance of knots. The evaluation of these
invariants requires a factorization of the Alexander polynomial of the knot,
and is therefore in practice often hard to realize. We thus propose the study
of an alternative set of invariants of algebraic concordance - the rational
Witt classes of knots. Though these are rather weaker invariants than those
defined by Levine, they have the advantage of lending themselves to quite
manageable computability. We illustrate this point by computing the rational
Witt classes of all pretzel knots. We give many examples and provide
applications to obstructing sliceness for pretzel knots. We also obtain
explicit formulae for the determinants and signatures of all pretzel knots.
  This article is dedicated to Jerry Levine and his lasting mathematical
legacy; on the occasion of the conference "Fifty years since Milnor and Fox"
held at Brandeis University on June 2-5, 2008.
