For formal multivariate power series $\varphi(x)$ an inversion formula of the
form $$ \varphi^{-1}(x)=x +\sum_{m=1}^{\infty}\sum_{k=0}^m (-1)^k(m
k)\varphi^{\circ k}(x) is offered$$.
In this paper for power series in many (real or complex)variables a radius of
(absolute) convergence is offered. This radius can be evaluated by a formula
similar to Cauchy-Hadamard formula and in one variable case they are same.
In this paper polynomial maps are represented by the use of matrices whose
entries are numbered by pair of multiindices. A new product of such matrices is
introduced. By the use of this and ordinary product of matrices the matrix
representation of composition of polynomial maps is given. A norm of such
matrices, which coincides with Bombieri's norm of a polynomial in a particular
case, is defined and investigated as well. A generalization of Bombieri's
inequality is offered.