 Isomorphism of formal group laws

An isomorphism from formal group law $F$ to $G$ over $R$ is a formal series $h(T)=a_{1}T+a_2{T}^2+\cdots$ with $a_1\in R^{\ast }$ and $h(F(X,Y))=G(h(X),h(Y))$. Coefficient recursion gives a compositional inverse over $R$. An isomorphism of formal series need not converge on an arbitrary prescribed analytic domain; convergence must be checked separately.