Transformation theory and Christoffel formulas for matrix biorthogonal polynomials on the real line

In this paper transformations for matrix orthogonal polynomials in the real line are studied. The orthogonality is understood in a broad sense, and is given in terms of a nondegenerate continuous sesquilinear form, which in turn is determined by a quasidefinite matrix of bivariate generalized functions with a well defined support. The discussion of the orthogonality for such a sesquilinear form includes, among others, matrix Hankel cases with linear functionals, general matrix Sobolev orthogonality and discrete orthogonal polynomials with an infinite support. The first transformation considered is that of Geronimus type, two different methods are developed. A spectral one, based on the spectral properties of the perturbing polynomial, and constructed in terms of the second kind functions. Then, using spectral techniques and spectral jets, Christoffel-Geronimus formulas for the transformed polynomials and norms are presented. For this type of transformations, the paper also proposes an alternative method, which does not require of spectral techniques. A discussion on matrix spectral linear transformations is presented, as well. These transformations can be understood as rational perturbations of the matrix of bivariate generalized functions together with an addition of appropriate masses, determined these last ones by the spectral properties of the polynomial denominator. As for the Geronimus case, two techniques are applied, spectral and mixed spectral/nonspectral. Christoffel--Geronimus--Uvarov formulas are found with both approaches and some applications are given. The transformation theory is finally discussed in the context of the 2D non-Abelian Toda lattice and noncommutative KP hierarchies, understood as the theory of continuous transformations of quasidefinite sesquilinear forms. This approach allows for the finding of perturbed quasitau and Baker matrices.