Author’s summary: The well-known theory of “rational canonical form of an operator” describes the invariant factors, or elementary divisors, as a complete set of invariants of a similarity class of an operator on a finite-dimensional vector space $\Bbb V$ over a given field $\Bbb F$. A finer part of the theory is the contribution by Frobenius dealing with the structure of the centralizer of an operator. The viewpoint is that of finitely generated modules over a PID. In this paper we approach the issue from a “dynamic” viewpoint, as explained in the author’s paper [J. Ramanujan Math. Soc. 22, No. 1, 35--56 (2007;

Zbl 1181.22022)]. We also extend the theory to affine maps. The formulation is in terms of the action of the general linear group $\text{GL}(n)$, resp. the group of invertible affine maps $\text{GA}(n)$, on the semigroup of all linear, resp. affine, maps by conjugacy. The theory of rational canonical forms is connected with the orbits, and the Frobenius’ theory with the orbit-classes, of the action of $\text{GL}(n)$ on the semigroup of linear maps. We describe a parametrization of orbits and orbit-classes of both $\text{GL}(n)$- and $\text{GA}(n)$-actions, and also provide a parametrization of all affine maps themselves, which is independent of the choices of linear or affine co-ordinate systems. An important ingredient in these parametrizations is a certain flag. For a linear map $T$ on $\Bbb V$, let $Z_L(T)$ denote its centralizer associative $\Bbb F$-algebra, and $Z_L(T)^*$the multiplicative group of invertible elements in $Z_L(T)$. In this situation, we associate a canonical, maximal, $Z_L(T)$-invariant flag, and precisely describe the orbits of $Z_L(T)^*$ on $\Bbb V$. Using this approach, we strengthen the classical theory in a number of ways.