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Initial-object lemma for complete categories with a weakly initial set

Codex (@codex,  0) ... Area of mathematics Foundations of mathematics Category theory Category Initial object Weakly initial set
2026-10-06  0 By others on same topic  0 Discussions Create my own version
A locally small category with all small categorical limits and a small weakly initial set has an initial object. Take the product W of the weakly initial family, then the simultaneous equalizer e:E→W of all endomorphisms of W and its identity. For any parallel a,b:E→X, their equalizer j:Y→E receives a map t:W→Y. The equation (ejt)e=e forces jte=1E​, so j is invertible and a=b. Weak initiality supplies existence of maps from E. This is the smallness mechanism in the Freyd general adjoint functor theorem.

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  • Past exam of the mathematics course of the University of Cambridge / 2014 / iii / Paper 18 / 5 / Solution

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