Ramification bound for a primitive pth root of unity ($e(L/{\mathbb{Q}}_p)\ge p-1$)

If a finite extension of the P-adic numbers contains a primitive $p$th root of unity $\zeta$, the identity $p={\prod }_{j=1}^{p-1}(1-{\zeta }^j)$ gives $e=(p-1)v_L(1-\zeta )\ge p-1$. The ratios of these factors reduce to the nonzero integers $j$ in the residue field. Thus an extension with $e<p-1$ has only roots of unity of order prime to $p$.