nLab
family of sets

A family of sets consists of an index set II and, for each element kk of II, a set S kS_k.

Given II, a set FF and a function p:FIp\colon F \to I, we get a family of sets by defining S kS_k to be the preimage p *(k)p^*(k).

Conversely, given a family of sets, let FF be the disjoint union

kS k={(k,x)kI,xS k} \biguplus_k S_k = \{ (k,x) | k \in I, x \in S_k \}

and let f(k,x)f(k,x) be kk.

(We should talk about ways to formalise this concept in various forms of set theory and when the latter construction above requires the axiom of collection.)

Revised on January 15, 2011 06:00:41 by Toby Bartels (98.19.56.183)