Functorial maps and weak parities
  Functorial maps and weak parities are equivalent descriptions of rules of substitution virtual crossings for classical in diagrams of a knot in a way compatible with Reidemeister moves. We introduce the notion of maximal weak parity and describe it for knots in a given closed oriented surface. This weak parity defines a projection from virtual knots to classical knots. 
Igor Nikonov
math.GT