  Every continuous-time flow on a topological space has associated to it a
Koopman operator, which operates by time-shifts on various spaces of functions,
such as $C^r$, $L^2$, or functions of bounded variation. An eigenfunction of
the vector field (and thus for the Koopman operator) can be viewed as an
$S^1$-valued function, which also plays the role of a semiconjugacy to a rigid
rotation on $S^1$. This notion of Koopman eigenfunctions will be generalized to
Lie-group valued eigenfunctions, and we will discuss the dynamical aspects of
these functions. One of the tools that will be developed to aid the discussion,
is a concept of exterior derivative for Lie group valued functions, which
generalizes the notion of the differential $df$ of a real valued function $f$.
The extended notion of Koopman eigenfunctions utilizes a geometric property of
usual eigenfunctions. We show that the generalization in a geometric sense can
be used to reveal fundamental properties of usual Koopman eigenfunctions, such
as their behavior under time-rescaling, and as submersions.
