Some excellent websites here.
https://encyclopediaofmath.org/wiki/Main_Page
https://plato.stanford.edu/entries/set-theory
https://betterexplained.com/archives
https://gcc.edu/dept/math/faculty/BancroftED/buscalc/index.php
https://sophisticatedprimate.com/math/rnr
https://dlmf.nist.gov](https://dlmf.nist.gov/
https://en.wikipedia.org/wiki/List_of_integrals
https://tutorial.math.lamar.edu](https://tutorial.math.lamar.edu/
https://openstax.org/subjects/math
https://sfu.ca/math-coursenotes
https://calculus.nipissingu.ca
https://whitman.edu/mathematics/calculus_online
https://clarku.edu/faculty/djoyce/trig
https://sophisticatedprimate.com
https://eeweb.com/tools/math-help