  Viewing a bivariate polynomial f in R[x,t] as a family of univariate
polynomials in t parametrized by real numbers x, we call f real rooted if this
family consists of monic polynomials with only real roots. If f is the
characteristic polynomial of a symmetric matrix with entries in R[x], it is
obviously real rooted. In this article the converse is established, namely that
every real rooted bivariate polynomial is the characteristic polynomial of a
symmetric matrix over the univariate real polynomial ring. As a byproduct we
present a purely algebraic proof of the Helton-Vinnikov Theorem which solved
the 60 year old Lax conjecture on the existence of definite determinantal
representation of ternary hyperbolic forms.
