On pg 70 in Armstrong's Basic Topology book, the author writes:
" We introduce the disjoint union of spaces and the function which when restricted to either or is just the inclusion in . This function is important for our purposes because:
a) it is continuous
b) the composition is continuous if and only if both and are continuous. "
The author does not mention anything about disjoint union before this. I also checked the index at the back of the book and did not find it. I found one definition of disjoint union on wiki. If I go by that definition then is not even a subset of so I don't know what the author is trying to say when he writes '... the function which when restricted to either or ...'
Does anybody have a clue what is the meaning of disjoint union intended here and what this function might be?