Abstract. We investigate rational G-modules M for a linear algebraic group G over an algebraically closed field k of characteristic p > 0 using filtrations by sub-coalgebras of the coordinate algebra k[G] of G. Even in the special case of the additive group Ga, interesting structures and examples are revealed. The "degree" filtration we consider for unipotent algebraic groups leads to a "filtration by exponential degree" applicable to rational G modules for any linear algebraic group G of exponential type; this filtration is defined in terms of 1-parameter subgroups and is related to support varieties introduced recently by the author for such rational G-modules. We formulate in terms of this filtration a necessary and sufficient condition for rational injectivity for rational G-modules. Our investigation leads to the consideration of two new classes of rational G-modules: those that are "mock injective" and those that are "mock trivial".
IntroductionBeginning with the very special case of the additive group G a , we consider the filtration by degree on rational G a -modules which enables us to better understand the intriguing category (G a -M od) of rational G a -modules. This filtration leads to a similarly defined filtration by degree on rational U N -modules, where U N ⊂ GL N is the closed subgroup of strictly upper triangular matrices, and determines a filtration on rational U -modules for a closed linear subgroup U ⊂ U N . We then initiate the study of a less evident filtration on rational G-modules for G a linear algebraic group of exponential type. For rational U N -modules for the unipotent algebraic group U N , this filtration of exponential degree is a comparable to the more elementary filtration by degree we first consider. Throughout, we fix an algebraic closed field k of characteristic p > 0 and consider (smooth) linear algebraic groups over k together with their rational actions on k-vector spaces.In some sense, this paper is a sequel to the author's recent paper [4] in which a theory of support varieties M → V (G) M was constructed for rational G-modules. The construction of the filtration by exponential degreeuses restrictions of M to 1-parameter subgroups G a → G, and thus is based upon actions of G on M at p-unipotent elements of G. The role of 1-parameter subgroups to study rational G-modules was introduced in [3]; the property of p-unipotent degree introduced in [3, 2.5] is the precursor to our filtration by exponential degree. The origins of this approach to filtrations lie in considerations of support varieties