NIST Digital Library of Mathematical Functions

Start with the index or notation guide when a formula uses unfamiliar symbols. Chapters are organized by function families, such as Bessel or gamma functions, and by mathematical methods. This makes the library useful beside an advanced mathematics or physics text. Search within that structure when you need a particular identity rather than an introductory course.

Cite the DLMF release and use the permalink for the exact equation or table. Its information box supplies that link, which is more useful to a reader than the homepage alone.