Barsotti-Tate groups and representations of
the fundamental group scheme -- Marco Andrea Garuti

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On a scheme of characteristic p>0, p-adic local systems (p-adic
representations of the fundamental group) constitute only a
small part the "right" category of coefficients.  There are
many generalizations (BT - or p-divisible - groups ,
F-(iso)crystals,...) none of which has an interpretation in
terms of fundamental groups.  We will show that BT groups which
are locally constant in the flat topology can be interpreted as
representations of the fundamental group scheme (classifying
finite flat torsors on the base scheme) and that this category
has many interesting propreties.