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For knots in $S^3$, it is well-known that the Alexander polynomial of aribbon knot factorizes as $f(t)f(t^{-1})$ for some polynomial $f(t)$. Bycontrast, the Alexander polynomial of a ribbon $2$-knot is not even symmetricin general. Via an alternative notion of ribbon $2$-knots, we give atopological condition on a $2$-knot that implies the factorization of theAlexander polynomial.

A Fox-Milnor theorem for the Alexander polynomial of knotted $2$-spheres in $S^4$