retractable algebra
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Section (category theory) — In category theory, a branch of mathematics, a section (or coretraction) is a right inverse of a morphism. Dually, a retraction (or retract) is a left inverse. In other words, if and are morphisms whose composition is the identity morphism on Y,… … Wikipedia
Ruler — For other uses, see Ruler (disambiguation). A variety of rulers A 2 meter … Wikipedia
