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Definition df-pnf 7956
Description: Define plus infinity. Note that the definition is arbitrary, requiring only that +oo be a set not in  RR and different from -oo (df-mnf 7957). We use  ~P
U. CC to make it independent of the construction of  CC, and Cantor's Theorem will show that it is different from any member of 
CC and therefore  RR. See pnfnre 7961 and mnfnre 7962, and we'll also be able to prove +oo  =/= -oo.

A simpler possibility is to define +oo as  CC and -oo as  { CC }, but that approach requires the Axiom of Regularity to show that +oo and -oo are different from each other and from all members of  RR. (Contributed by NM, 13-Oct-2005.) (New usage is discouraged.)

Assertion
Ref Expression
df-pnf  |- +oo  =  ~P U. CC

Detailed syntax breakdown of Definition df-pnf
StepHypRef Expression
1 cpnf 7951 . 2  class +oo
2 cc 7772 . . . 4  class  CC
32cuni 3796 . . 3  class  U. CC
43cpw 3566 . 2  class  ~P U. CC
51, 4wceq 1348 1  wff +oo  =  ~P U. CC
Colors of variables: wff set class
This definition is referenced by:  pnfnre  7961  mnfnre  7962  pnfxr  7972
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