Isomorphism of formal group laws
= Isomorphism of formal group laws
{title2=$h(F(X,Y))=G(h(X),h(Y))$}
An isomorphism from <formal group law> $F$ to $G$ over $R$ is a formal series $h(T)=a_1T+a_2T^2+\cdots$ with $a_1\in R^*$ 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.