[Help-bash] Arithmetic evaluation / expansion question

[Bruce Hohl]

An arithmetic evaluation / expansion question please:  the man page
indicates that base 10 is the default, and null or unset variables evaluate
to zero.

Are strings also evaluated to zero within $(( ))?
(Except those beginning with 0 which are octals.)

$ pd=abc
$ echo $pd
abc
$ echo $((pd))
0