Theorem orandc 941

Description: Disjunction in terms of conjunction (De Morgan's law), for decidable propositions. Compare Theorem *4.57 of [WhiteheadRussell] p. 120. (Contributed by Jim Kingdon, 13-Dec-2021.)

Assertion

Ref	Expression
orandc	|- ( (DECID ph /\ DECID ps ) -> ( ( ph \/ ps ) <-> -. ( -. ph /\ -. ps ) ) )

Proof of Theorem orandc

Step	Hyp	Ref	Expression
1		pm4.56 781	. 2 |- ( ( -. ph /\ -. ps ) <-> -. ( ph \/ ps ) )
2		dcn 843	. . . 4 |- (DECID ph -> DECID -. ph )
3		dcn 843	. . . 4 |- (DECID ps -> DECID -. ps )
4		dcan 935	. . . 4 |- ( (DECID -. ph /\ DECID -. ps ) -> DECID ( -. ph /\ -. ps ) )
5	2, 3, 4	syl2an 289	. . 3 |- ( (DECID ph /\ DECID ps ) -> DECID ( -. ph /\ -. ps ) )
6		dcor 937	. . . 4 |- (DECID ph -> (DECID ps -> DECID ( ph \/ ps ) ) )
7	6	imp 124	. . 3 |- ( (DECID ph /\ DECID ps ) -> DECID ( ph \/ ps ) )
8		con2bidc 876	. . 3 |- (DECID ( -. ph /\ -. ps ) -> (DECID ( ph \/ ps ) -> ( ( ( -. ph /\ -. ps ) <-> -. ( ph \/ ps ) ) <-> ( ( ph \/ ps ) <-> -. ( -. ph /\ -. ps ) ) ) ) )
9	5, 7, 8	sylc 62	. 2 |- ( (DECID ph /\ DECID ps ) -> ( ( ( -. ph /\ -. ps ) <-> -. ( ph \/ ps ) ) <-> ( ( ph \/ ps ) <-> -. ( -. ph /\ -. ps ) ) ) )
10	1, 9	mpbii 148	1 |- ( (DECID ph /\ DECID ps ) -> ( ( ph \/ ps ) <-> -. ( -. ph /\ -. ps ) ) )

Syntax hints:   -. wn 3   -> wi 4   /\ wa 104   <-> wb 105   \/ wo 709  DECID wdc 835

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 710

This theorem depends on definitions:  df-bi 117  df-stab 832  df-dc 836

This theorem is referenced by:  gcdaddm  12176