International Journal of Mathematics and Mathematical Sciences
Volume 2012 (2012), Article ID 254791, 22 pages
Research Article

Subring Depth, Frobenius Extensions, and Towers

Departamento de Matematica, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal

Received 24 March 2012; Accepted 23 April 2012

Academic Editor: Tomasz Brzezinski

Copyright © 2012 Lars Kadison. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


The minimum depth 𝑑 ( 𝐵 , 𝐴 ) of a subring 𝐵 𝐴 introduced in the work of Boltje, Danz and Külshammer (2011) is studied and compared with the tower depth of a Frobenius extension. We show that 𝑑 ( 𝐵 , 𝐴 ) < ∞ if 𝐴 is a finite-dimensional algebra and 𝐵 𝑒 has finite representation type. Some conditions in terms of depth and QF property are given that ensure that the modular function of a Hopf algebra restricts to the modular function of a Hopf subalgebra. If 𝐴 𝐵 is a QF extension, minimum left and right even subring depths are shown to coincide. If 𝐴 𝐵 is a Frobenius extension with surjective Frobenius, homomorphism, its subring depth is shown to coincide with its tower depth. Formulas for the ring, module, Frobenius and Temperley-Lieb structures are noted for the tower over a Frobenius extension in its realization as tensor powers. A depth 3 QF extension is embedded in a depth 2 QF extension; in turn certain depth 𝑛 extensions embed in depth 3 extensions if they are Frobenius extensions or other special ring extensions with ring structures on their relative Hochschild bar resolution groups.