André–Quillen cohomology
Introduction In commutative algebra, '''André–Quillen cohomology''' is a theory of cohomology for commutative rings which is closely related to the cotangent complex. The first three cohomology groups were introduced by  and are sometimes called '''Lichtenbaum–Schlessinger functors''' ''T''0, ''T''1, ''T''2, and the higher groups were defined independently by  and  using methods of homotopy theory. It comes with a parallel homology theory called '''André–Quillen homology'''.
Motivation Let ''A'' be a commutative ring, ''B'' be an ''A''-algebra, and ''M'' be a ''B''-module. The André–Quillen cohomology groups are the derived functors of the derivation functor Der''A''(''B'', ''M''). Before the general definitions of André and Quillen, it was known for a long time that given morphisms of commutative rings  and a ''C''-module ''M'', there is a three-term exact sequence of derivation modules::This term can be extended to a six-term exact sequence using the functor Exalcomm of extensions of commutative algebras and a nine-term exact sequence using the Lichtenbaum–Schlessinger functors. André–Quillen cohomology extends this exact sequence even further. In the zeroth degree, it is the module of derivations; in the first degree, it is Exalcomm; and in the second degree, it is the second degree Lichtenbaum–Schlessinger functor.
Definition Let ''B'' be an ''A''-algebra, and let ''M'' be a ''B''-module. Let ''P'' be a simplicial cofibrant ''A''-algebra resolution of ''B''. André notates the ''q''th cohomology group of ''B'' over ''A'' with coefficients in ''M'' by , while Quillen notates the same group as . The ''q''th '''André–Quillen cohomology group''' is::Let  denote the relative cotangent complex of ''B'' over ''A''. Then we have the formulas:::
See also * Cotangent complex* Deformation Theory* Exalcomm
References *** **=== Generalizations ===*  André–Quillen cohomology of commutative S-algebras* Homology and Cohomology of E-infinity ring spectra