A researcher interested in the relationship between music and math, specifically as they relate to mathematician and philosopher Pythagoras, might start by simply looking up Pythagoras. This will undoubtedly turn up results, but chances are none of those results will focus solely on his theories on music. Exploring subject headings and broader and narrower terms can help come up with more specific terms for search for.

A few general subject headings to start include:

Mathematics--Miscellanea
Music and science
Pythagoras and Pythagorean school

More specific subject headings include:

Harmony of the spheres
Music--Philosophy and aesthetics
Tuning