On infinite order differential operators in fractional viscoelasticity
  In this paper we discuss some general properties of viscoelastic models
defined in terms of constitutive equations involving infinitely many
derivatives (of integer and fractional order). In particular, we consider as a
working example the recently developed Bessel models of linear viscoelasticiy
that, for short times, behave like fractional Maxwell bodies of order $1/2$.
