Let "K" be a knot in the 3-sphere. Let "X" be the infinite cyclic cover of the knot complement of "K". This covering can be obtained by cutting the knot complement along a Seifert surface of "K" and gluing together infinitely many copies of the resulting manifold with boundary in a cyclic manner. There is a covering transformation "t" acting on "X". Consider the first homology (with integer coefficients) of "X", denoted formula_1. The transformation "t" acts on the homology and so we can consider formula_1 a module over the ring of Laurent polynomials formula_3. This is called the Alexander invariant or Alexander module.