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Definition df-nn 9926
Description: The natural numbers of analysis start at one (unlike the ordinal natural numbers, i.e. the members of the set  om, df-om 4779, which start at zero). This is the convention used by most analysis books, and it is often convenient in proofs because we don't have to worry about division by zero. See nnind 9943 for the principle of mathematical induction. See dfnn2 9938 for a slight variant. See df-n0 10147 for the set of nonnegative integers  NN0 starting at zero. See dfn2 10159 for  NN defined in terms of  NN0.

This is a technical definition that helps us avoid the Axiom of Infinity in certain proofs. For a more conventional and intuitive definition ("the smallest set of reals containing 
1 as well as the successor of every member") see dfnn3 9939. (Contributed by NM, 10-Jan-1997.)

Assertion
Ref Expression
df-nn  |-  NN  =  ( rec ( ( x  e.  _V  |->  ( x  +  1 ) ) ,  1 ) " om )

Detailed syntax breakdown of Definition df-nn
StepHypRef Expression
1 cn 9925 . 2  class  NN
2 vx . . . . 5  set  x
3 cvv 2892 . . . . 5  class  _V
42cv 1648 . . . . . 6  class  x
5 c1 8917 . . . . . 6  class  1
6 caddc 8919 . . . . . 6  class  +
74, 5, 6co 6013 . . . . 5  class  ( x  +  1 )
82, 3, 7cmpt 4200 . . . 4  class  ( x  e.  _V  |->  ( x  +  1 ) )
98, 5crdg 6596 . . 3  class  rec (
( x  e.  _V  |->  ( x  +  1
) ) ,  1 )
10 com 4778 . . 3  class  om
119, 10cima 4814 . 2  class  ( rec ( ( x  e. 
_V  |->  ( x  + 
1 ) ) ,  1 ) " om )
121, 11wceq 1649 1  wff  NN  =  ( rec ( ( x  e.  _V  |->  ( x  +  1 ) ) ,  1 ) " om )
Colors of variables: wff set class
This definition is referenced by:  nnexALT  9927  peano5nni  9928  1nn  9936  peano2nn  9937
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