Abstract
Let Λ be the four subspace algebra. We show that for any Λ-module M there exists an algorithm (up to the problem of finding roots of the so-called characteristic polynomial of M) with relatively low polynomial complexity of determining multiplicities of all direct summands of M. Moreover, we give a fully algorithmic criterion for deciding if two Λ-modules M and N are isomorphic.
ACKNOWLEDGMENT
The author was supported by the Research Grant N N201 542138 of the Polish Ministry of Science and Higher Education.
Notes
The proofs for all cases are available at the web address http://www-users.mat.umk.pl/ ∼izydor/prace/Proposition3.3.pdf
Communicated by D. Zacharia.