# Alexander-Conway polynomial

$$\Delta_{L_+}-\Delta_{L_-}=z\Delta_{L_0}$$
and the initial condition $\Delta_{T_1}=1$, where $T_1$ is the trivial knot (cf. also Knot theory). For $z=\sqrt t-1/\sqrt t$ one gets the original Alexander polynomial (defined only up to $\pm t^i$).