Problem::Conway Polynomial of Knot

$$\mathrm{Show} \, \, \mathrm{that} \, \, \nabla_K (z) = a_0 + a_2 z^2 + a_4 z^4 + \cdots + a_{2n} z^{2n} \quad \forall K \in \{\mathrm{knot}\}, \; a_{2i}\not= 0,\; (1 \le i \le n) $$