# What does this symbol mean?

This one
in this context

I never understood what it means and I can't find its meaning...

D H
Staff Emeritus
It means union. In this particular case, it is the union, for all subsets G of the set T, of the sets of ordered pairs of the elements constructible from the set G.

In other words, it is the set of all mappings (of functions) of T into itself.

Last edited: