Published online by Cambridge University Press: 16 May 2018
This paper studies the combinatorics of lattice congruences of the weak order on a finite Weyl group
$W$
, using representation theory of the corresponding preprojective algebra
$\unicode[STIX]{x1D6F1}$
. Natural bijections are constructed between important objects including join-irreducible congruences, join-irreducible (respectively, meet-irreducible) elements of
$W$
, indecomposable
$\unicode[STIX]{x1D70F}$
-rigid (respectively,
$\unicode[STIX]{x1D70F}^{-}$
-rigid) modules and layers of
$\unicode[STIX]{x1D6F1}$
. The lattice-theoretically natural labelling of the Hasse quiver by join-irreducible elements of
$W$
is shown to coincide with the algebraically natural labelling by layers of
$\unicode[STIX]{x1D6F1}$
. We show that layers of
$\unicode[STIX]{x1D6F1}$
are nothing but bricks (or equivalently stones, or 2-spherical modules). The forcing order on join-irreducible elements of
$W$
(arising from the study of lattice congruences) is described algebraically in terms of the doubleton extension order. We give a combinatorial description of indecomposable
$\unicode[STIX]{x1D70F}^{-}$
-rigid modules for type
$A$
and
$D$
.
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