This book is about the representation theory of commutative local
rings, specifically the study of maximal Cohen-Macaulay modules over
Cohen-Macaulay local rings.
The guiding principle of representation theory, broadly speaking,
is that we can understand an algebraic structure by studying the sets
upon which it acts. Classically, this meant understanding finite groups
by studying the vector spaces they act upon; the powerful tools of lin-
ear algebra can then be brought to bear, revealing information about
the group that was otherwise hidden. In other branches of represen-
tation theory, such as the study of finite-dimensional associative alge-
bras, sophisticated technical machinery has been built to investigate
the properties of modules, and how restrictions on modules over a ring
restrict the structure of the ring.
The representation theory of maximal Cohen-Macaulay modules
began in the late 1970s and grew quickly, inspired by three other ar-
eas of algebra. Spectacular successes in the representation theory of
finite-dimensional algebras during the 1960s and 70s set the standard
for what one might hope for from a representation theory. In partic-
ular, this period saw: P. Gabriel’s introduction of the representations
of quivers and his theorem that a quiver has finite representation type
if and only if it is a disjoint union of ADE Coxeter-Dynkin diagrams;
M. Auslander’s influential Queen Mary notes applying his work on
functor categories to representation theory; Auslander and I. Reiten’s
foundational work on AR sequences; and key insights from the Kiev
school, particularly Y. Drozd, L. A. Nazarova, and A. V. Ro˘ ıter. All
these advances continued the work on finite representation type be-
gun in the 1940s and 50s by T. Nakayama, R. Brauer, R. Thrall, and
J. P. Jans. Secondly, the study of lattices over orders, a part of integral
representation theory, blossomed in the late 1960s. Restricting atten-
tion to lattices rather than arbitrary modules allowed a rich theory to
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