For topics directly related to set theory and formal logic.
“Zermelo–Fraenkel set theory is a first-order axiomatic set theory. Under this name are known two...Zermelo–Fraenkel set theory
“Peano arithmetic (also known as first-order arithmetic) is a first-order axiomatic theory over...Peano arithmetic
“Second-order arithmetic (also known as Z2 or as\(\Pi_\infty^1-CA\)) is a first-order theory of...Second-order arithmetic
“In set theory, the cardinal numbers (or just cardinals) are equivalence classes defined by the...Cardinal