We introduce an operation on modules over an $F$-finite ring of
characteristic $p$. We call this operation \emph{tight interior}. While it
exists more generally, in some cases this operation is equivalent to the Matlis
dual of tight closure. Moreover, the interior of the ring itself is simply the
big test ideal. We directly prove, without appeal to tight closure, results
analogous to persistence, colon capturing, and working modulo minimal primes,
and we begin to develop a theory dual to phantom homology.
Using our dual notion of persistence, we obtain new and interesting
transformation rules for tight interior, and so in particular for the test
ideal, which complement the main results of a recent paper of the second author
and K. Tucker. Using our theory of phantom homology, we prove a vanishing
theorem for maps of Ext. We also compare our theory to M. Blickle's notion of
Cartier modules, and in the process, we prove new existence results for
Blickle's test submodule. Finally, we apply the theory we developed to the
study of test ideals in non-normal rings, proving that the finitistic test
ideal coincides with the big test ideal in some cases.Comment: References added and other minor changes. To appear in the Nagoya
Mathematical Journa