I wouldn't use "pertinacious" if I were you. It's very obscure - listed in a few dictinoaries, and probably in a thesaurus (is that where you found it?), but hardly used at all.
A thesaurus lists words that are
related in meaning; the closeness of the relation is variable, according to context. They are not always synonymous. "Determined' and "indefatigable", for example, can often be used in similar contexts; a person who is
determined to do something may be
indefatigable. But they don't have the same meaning.
Determined denotes a state of mind -
determined: Definition, Synonyms and Much More from Answers.com Indefatigable refers to tirelessness -
indefatigable: Definition, Synonyms and Much More from Answers.com
b