We consider random matrices that have invariance properties under the action
of unitary groups (either a left-right invariance, or a conjugacy invariance),
and we give formulas for moments in terms of functions of eigenvalues. Our main
tool is the Weingarten calculus. As an application to statistics, we obtain new
formulas for the pseudo inverse of Gaussian matrices and for the inverse of
compound Wishart matrices.
We study asymptotics of representations of the unitary groups U(n) in the
limit n\to\infty and we show that in many aspects they behave like large random
matrices. In particular, we show that the highest weight of a random
irreducible component in the Kronecker tensor product of two irreducible
representations behaves asymptotically in the same way as the spectrum of the
sum of two large random matrices with prescribed eigenvalues.
We consider integrals of type $\int_{O_n}u_{11}^{a_1}...
u_{1n}^{a_n}u_{21}^{b_1}... u_{2n}^{b_n} du$, with respect to the Haar measure
on the orthogonal group. We establish several remarkable invariance properties
satisfied by such integrals, by using combinatorial methods. We present as well
a general formula for such integrals, as a sum of products of factorials.