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In the thesis we introduce and study cohomology theories of <i>A</i>_∞-algebras.  There are two theories: Hochschild cohomology and Shukla cohomology.  Hochschild cohomology classifies split extensions of <i>A</i>_∞-algebras, while Shukla cohomology is introduced to classify all extensions.  Standard theorems are generalized for introduced cohomology theories.  Thanks to the properties of <i>A</i>_∞-algebras mentioned theorems are formulated in much more general form rather than for standard cases.  This generality implies nontrivial results if we substitute instead of <i>A</i>_∞-algebras an associative algebra.

Let G be a semisimple simply connected linear algebraic group over an algebraically closed field k of characteristic p. In [11], Donkin gave a recursive description for the characters of cohomology of line bundles on the flag variety G/B with G = SL3. In chapter 2 of this thesis we try to give a non recursive description for these characters. In chapter 3, we give the first step of a version of formulae in [11] for G = G2. In his famous paper [7], Demazure introduced certain indecomposable modules and used them to give a short proof of the Borel-Weil-Bott theorem (characteristic zero). In chapter 5 we give the cohomology of these modules. In a recent paper [17], Doty introduces the notion of r−minuscule weight and exhibits a tensor product factorization of a corresponding tilting module under the assumption p >= 2h − 2, where h is the Coxeter number. In chapter 4, we remove the restriction on p and consider some variations involving the more general notion of (p,r)−minuscule weights.

Στην παρούσα εργασία παρουσιάζουμε βασικές έννοιες του αντικειμένου της Ομολογιακής Άλγεβρας,όπως αυτές των μακρών ακριβών ακολουθιών, τις επεκτάσεις των modules και τις ομάδες Ext. Στη συνέχεια παρουσιάζουμε τις επεκτάσεις των ομολογιακών και συνομολογιακών συναρτητών, τους παράγωγους συναρτητές, που προκύπτουν μέσω προβολικών και ενριπτικών επιλύσεων αντικειμένων Αβελιανών κατηγοριών. Τέλος χαρακτηρίζουμε τους παράγωγους συναρτητές μέσω της καθολικής τους ιδιότητας. / In this work we present the basic concepts of Homological Algebra, such as the long exact sequences, the extensions of modules and the Ext-groups.Further we present the extensions of the homology and cohomology functors, the derived functors that arise through projective and injective resolutions from objects of an Abelian category. Finally we characterize derived functors through their universal property.