NIST Digital Library of Mathematical Functions
Eq World
Abundant information on solving many types of equations, from the Russian Foundation for Basic Research.
Handbook of Mathematical Formulas and Integrals
QA 47 J38 2000 (Reference, Archer Library 1st floor)
CRC Standard Mathematical Tables and Formulae (31st ed.)
QA47 M315 2003 (Reference, Archer Library 1st floor)
Table of Integrals, Series, and Products (6th ed.)
QA55 G6613 2000 (Reference, Archer Library 1st floor)
A huge tome (over a thousand pages) of every integral, series, and product imaginable.
The Mathematics Subject Classification can be very helpful when searching in MathSciNet. You can search for subject categories within the classification when you are in MathSciNet (under the tab labeled Free Tools), or you can browse the entire subject classification at https://mathscinet.ams.org/mathscinet/msc/pdfs/classifications2020.pdf.
Encyclopedia of Mathematics
This major reference work began as a traditional encyclopedia, and has since been built upon with wiki-style edits and new contributions from users.
International Encyclopedia of Statistical Science
Encyclopedia of Statistical Sciences (9 vol's + supplement)
QA 276.14 E5 (4th floor).
This is a major reference work with in-depth coverage of statistical topics.
International Encyclopedia of Statistics (2 vol's)
HA 17 I63 (Reference, Archer Library 1st floor)
Encyclopedic Dictionary of Mathematics (4 vol's)
QA 5 I8313 1987 (Archer Library 4th floor)
CRC Concise Encyclopedia of Mathematics
QA 5 W45 2003 (Reference, Archer Library 1st floor)
The VNR Concise Encyclopedia of Mathematics
QA 40 V18 1989 (Archer Library 4th floor)