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Kuperberg [Algebr. Geom. Topol. 3 (2003) 587-591] has shown that a virtual knot
diagram corresponds (up to generalized Reidemeister moves) to a unique embedding
in a thickened surface of minimal genus. If a virtual knot diagram is equivalent to
a classical knot diagram then this minimal surface is a sphere. Using this
result and a generalised bracket polynomial, we develop methods that may
determine whether a virtual knot diagram is non-classical (and hence non-trivial).
As examples we show that, except for special cases, link diagrams with a
single virtualization and link diagrams with a single virtual crossing are
non-classical.
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