  Let End(V) denote the ring of all linear transformations of an arbitrary
k-vector space V over a field k. We define a subset X of End(V) to be
"triangularizable" if V has a well-ordered basis such that X sends each vector
in that basis to the subspace spanned by basis vectors no greater than it. We
then show that an arbitrary subset of End(V) is "strictly" triangularizable
(defined in the obvious way) if and only if it is topologically nilpotent. This
generalizes the theorem of Levitzki that every nilpotent semigroup of matrices
is triangularizable. We also give a description of the triangularizable
subalgebras of End(V), which generalizes of a theorem of McCoy classifying
triangularizable algebras of matrices over algebraically closed fields.
