Abstract
The HC-move was defined as an unknotting operation of a ribbon 2-knot. Representing a ribbon 2-knot by a virtual arc, we see that the HC-move corresponds to one of the "forbidden moves", which unknot every virtual knot. Using this, we consider the α2-invariant of a ribbon 2-knot and a relation between the Δ-move for a 1-knot and the HC-move for its spun 2-knot.
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