Hilbert's theorem 90 ($H^1(K,{\overline{K}}^{\ast })=0$)

For a finite Galois extension $L/K$, multiplicative Hilbert theorem 90 says $H^1(\mathrm{Gal}(L/K),L^{\ast })=0$. A multiplicative cocycle has the form $c_{\sigma }=\sigma (b)/b$. One proof forms a nonzero sum $A={\sum }_{\sigma }{c}_{\sigma }\sigma (a)$, using linear independence of field automorphisms. Its transformation law is $\tau A=c_{\tau }^{-1}A$, so $b=A^{-1}$ gives $c_{\tau }=\tau (b)/b$. Passing to the algebraic closure gives $H^1(K,{\overline{K}}^{\ast })=0$, so the multiplicative Kummer sequence identifies $H^1(K,{\mu }_m)$ with $K^{\ast }/K^{\ast m}$.