Pole Placement and Matrix Extension Problems: A Common Point of View

The paper studies a general inverse eigenvalue problem which generalizes many well studied pole placement and matrix extension problems. It is shown that the problem corresponds geometrically to a so-called central projection from some projective variety. The degree of this variety represents the number of solutions the inverse problem has in the critical dimension. We are able to compute this degree in many instances and we provide upper bounds in the general situation.