  While Liouvillian sequences are closed under many operations, simple examples
show that they are not closed under convolution, and the same goes for
d'Alembertian sequences. Nevertheless, we show that d'Alembertian sequences are
closed under convolution with rationally d'Alembertian sequences, and that
Liouvillian sequences are closed under convolution with rationally Liouvillian
sequences.
