Description: Define plus infinity.
Note that the definition is arbitrary, requiring
only that be
a set not in and
different from
(df-mnf 5460). We use   to make it
independent of the
construction of ,
and Cantor's Theorem will show that it is
different from any member of and therefore . See pnfnre 5468,
mnfnre 5469, and pnfnemnf 5509.
A simpler possibility is to define as and as
  , but that approach requires the Axiom of
Regularity to show
that and are different from each
other and from all
members of . |