nf4r - Intuitionistic Logic Explorer

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Theorem nf4r 1694

Description: If

is always true or always false, then variable

is effectively not free in

. The converse holds given a decidability condition, as seen at nf4dc 1693. (Contributed by Jim Kingdon, 21-Jul-2018.)

**Assertion**
Ref	Expression
nf4r	$\/$

**Proof of Theorem nf4r**
Step	Hyp	Ref	Expression
1		orcom 730	. . 3 $\/$ $\/$
2		alnex 1522	. . . 4
3	2	orbi2i 764	. . 3 $\/$ $\/$
4	1, 3	bitr4i 187	. 2 $\/$ $\/$
5		imorr 723	. . 3 $\/$
6		nf2 1691	. . 3
7	5, 6	sylibr 134	. 2 $\/$
8	4, 7	sylbir 135	1 $\/$

Colors of variables: wff set class

Syntax hints:

wn 3

wi 4 $\/$ wo 710

wal 1371

wnf 1483

wex 1515

This theorem was proved from axioms: ax-mp 5 ax-1 6 ax-2 7 ax-ia1 106 ax-ia2 107 ax-ia3 108 ax-in1 615 ax-in2 616 ax-io 711 ax-5 1470 ax-gen 1472 ax-ie2 1517 ax-4 1533 ax-ial 1557

This theorem depends on definitions: df-bi 117 df-tru 1376 df-fal 1379 df-nf 1484

This theorem is referenced by: (None)