  The analytical solution to the initial-boundary value problem for the cell
growth equation was given in the paper Zaidi A. A., Van Brunt B., Wake G.C.,
Solutions to an advanced functional partial differential equation of the
pantograph type, Proc. R. Soc. A 471: 20140947 (2015). In this note, we
simplify the arguments given in the paper mentioned above by using the theory
of operator semigroups.
