Home  18.013A  Chapter 1  Section 1.2 


To put a set in correspondence with another means to pair off elements of the first set with those of the other one.
For example we can put the set
$\{a,b,c\}$
in correspondence with
$\{1,2,3\}$
by pairing a with
$1,b$
with 2 and
$c$
with 3.
There is not a similar correspondence of
$\{a,b\}$
with
$\{1,2,3\}$
, although we can form a correspondence of
$\{a,b\}$
with a subset of
$\{1,2,3\}$
, for example, with
$\{2,3\}$
.

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