  We construct a Hopf algebra structure on the space of specified Feynman
graphs of a quantum field theory. We introduce a convolution product and a
semigroup of characters of this Hopf algebra with values in some suitable
commutative algebra taking momenta into account. We then implement the
renormalization described by A. Connes and D. Kreimer and the Birkhoff
decomposition for two renormalization schemes: the minimal subtraction scheme
and the Taylor expansion scheme.
