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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.    
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Please read these notes from the beginning to here and then sentences 4 and 5 of paragraph 7 of 3.

THE IDENTITY FUNCTION

begin quote ita-1-3-7-pg10 "
An example of a function that is one-one onto is, for any set   X,   the identity function   i : XX,   given by   i(x) = x   for all  xX.   (Note that what goes under the name "the function x" in elementary calculus is actually the identity function on the set of real numbers.)
"end quote

The parenthetical remark assumes Rosenlicht's prerequisites. Given the description of the identity function however, the parenthetical remark makes sense even without prior knowledge of the 'real' numbers.

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