Abstract: In this talk I will first present the classical
Cayley Tranformation for matrices. Then the generalization to
reductive groups, due to B. Kostant and me:
Each infinitesimally faithful representation of a reductive complex
connected algebraic group $G$ induces a dominant morphism $\Phi$ from
the group to its Lie algebra $\g$ by orthogonal projection in the
endomorphism ring of the representation space. The map $\Phi$
identifies the field $Q(G)$ of rational functions on $G$ with an
algebraic extension of the field $Q(\g)$ of rational functions on
$\g$. For the spin representation of $\on{Spin}(V)$ the map $\Phi$
essentially coincides with the classical Cayley transform. In general,
properties of $\Phi$ are established and these properties are applied
to deal with a separation of variables (Richardson) problem for
reductive algebraic groups: Find $\on{Harm}(G)$ so that for the
coordinate ring $A(G)$ of $G$ we have $A(G) = A(G)^G\otimes
\on{Harm}(G)$. As a consequence of a partial solution to this problem
and a complete solution for $SL(n)$ one has in general the equality
$[Q(G):Q(\g)] = [Q(G)^G:Q(\g)^G]$ of the degrees of extension
fields. Among other results, $\Phi$ yields (for the complex case) a
generalization, involving generic regular orbits, of the result of
Richardson showing that the Cayley map, when $G$ is semisimple,
defines an isomorphism from the variety of unipotent elements in $G$
to the variety of nilpotent elements in $\g$. In addition if $G$ is
semisimple the Cayley map establishes a diffeomorphism between the
real submanifold of hyperbolic elements in $G$ and the space of
infinitesimal hyperbolic elements in $\g$. Some examples are computed
in detail. In particular the classical Cayley tranformation for
orthogonal matrices, multiplied by a regular function (vanishing on
the poles) equal the Cayley map fort the spin representation.