I am more and more seeing "such-and-such and equivalent" (or "or equivalent") rather than "such-and-such and the equivalent." Are both usages correct, and are there any source materials that discuss either or both usages?
According to the British National Corpus, 'and equivalent' is more common. You can search here for examples:
[DAVIES] Variation in English Words and Phrases (VIEW)
However, 'equivalent' could be used as a noun or adjective, which would have an effect on whether to use the article or not.