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Definition df-nf 1551
Description: Define the not-free predicate for wffs. This is read " x is not free in  ph". Not-free means that the value of  x cannot affect the value of  ph, e.g., any occurrence of  x in  ph is effectively bound by a "for all" or something that expands to one (such as "there exists"). In particular, substitution for a variable not free in a wff does not affect its value (sbf 2059). An example of where this is used is stdpc5 1806. See nf2 1878 for an alternative definition which does not involve nested quantifiers on the same variable.

Not-free is a commonly used constraint, so it is useful to have a notation for it. Surprisingly, there is no common formal notation for it, so here we devise one. Our definition lets us work with the not-free notion within the logic itself rather than as a metalogical side condition.

To be precise, our definition really means "effectively not free," because it is slightly less restrictive than the usual textbook definition for not-free (which only considers syntactic freedom). For example,  x is effectively not free in the bare expression  x  =  x (see nfequid 1685), even though  x would be considered free in the usual textbook definition, because the value of  x in the expression  x  =  x cannot affect the truth of the expression (and thus substitution will not change the result).

This predicate only applies to wffs. See df-nfc 2512 for a not-free predicate for class variables. (Contributed by Mario Carneiro, 11-Aug-2016.)

Ref Expression
df-nf  |-  ( F/ x ph  <->  A. x
( ph  ->  A. x ph ) )

Detailed syntax breakdown of Definition df-nf
StepHypRef Expression
1 wph . . 3  wff  ph
2 vx . . 3  set  x
31, 2wnf 1550 . 2  wff  F/ x ph
41, 2wal 1546 . . . 4  wff  A. x ph
51, 4wi 4 . . 3  wff  ( ph  ->  A. x ph )
65, 2wal 1546 . 2  wff  A. x
( ph  ->  A. x ph )
73, 6wb 177 1  wff  ( F/ x ph  <->  A. x
( ph  ->  A. x ph ) )
Colors of variables: wff set class
This definition is referenced by:  nfi  1557  nfbii  1575  nfdv  1646  nfr  1769  nfd  1774  nfbidf  1782  19.9t  1787  nfnf1  1798  nfnd  1799  nfndOLD  1800  nfimd  1817  nfimdOLD  1818  nfnf  1857  nfnfOLD  1858  nf2  1878  drnf1  2008  drnf2  2009  sbnf2  2141  axie2  2363  xfree  23795  hbexg  27986  drnf1NEW7  28833  drnf2wAUX7  28836  drnf2w2AUX7  28839  drnf2w3AUX7  28842  sbnf2NEW7  28941  nfnfOLD7  29005  drnf2OLD7  29032
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