  Let G be a connected linear algebraic group over a number field k. We
establish an exact sequence describing the closure of the group G(k) of
rational points of G in the group of adelic points of G. This exact sequence
describes the defect of strong approximation on G in terms of the algebraic
Brauer group of G. In particular, we deduce from those results that the
integral Brauer-Manin obstruction on a torsor under the group G is the only
obstruction to the existence of an integral point on this torsor. We also
obtain a non-abelian Poitou-Tate exact sequence for the Galois cohomology of
the linear group G. The main ingredients in the proof of those results are the
local and global duality theorems for complexes of k-tori of length two and the
abelianization maps in Galois cohomology introduced by Borovoi.
