Maximal T-spaces of a free associative algebra

We study the lattice of T-spaces of a free associative k-algebra over a nonempty set. It is shown that when the field k is infinite, then the lattice has a maximum element, and that maximum element is in fact a T-ideal. In striking contrast, it is then proven that when the field k is finite, the lattice of T-spaces has infinitely many maximal elements (of which exactly two are T-ideals). Similar results are also obtained for the free unitary associative k-algebras. The proof is based on the observation that there is a natural bijection between the sets of maximal T-spaces of the free associative k-algebras over a nonempty set X and over a singleton set. This permits the transfer of results from the study of the lattice of T-spaces of the free associative k-algebra over a one-element set to the general case.