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begin quote ita"
CHAPTER I [:] Notions from Set Theory [...] 3. FUNCTIONS. "end quote
These pages assume that the reader has the book and is reading along.    
topic index     contents
Please read these notes from the beginning to here, and then paragraph 1 of 3.

DESCRIPTION OF FUNCTIONS

begin quote ita-1-3-1-pg8 "
If   X   and   Y   are sets, by a   function from   X   to   Y   [...] is meant a rule which associates with each element of   X   a definite element of   Y.
"end quote

In particular, looking forward to chapter two on the real number system, I believe ...

begin quote ita-1-3-1-pg8 "
[...] the essential thing is that given any [ANY] element of   X   there is associated, somehow, some definite element [ONE AND ONLY ONE ELEMENT] of   Y.
"end quote

on to reading 2