Description: Negated disjunction in terms of conjunction (De Morgan's law). Compare Theorem *4.56 of WhiteheadRussell p. 120. (Contributed by NM, 3-Jan-1993) (Proof shortened by Andrew Salmon, 7-May-2011)
| Ref | Expression | ||
|---|---|---|---|
| Assertion | ioran | |- ( -. ( ph \/ ps ) <-> ( -. ph /\ -. ps ) ) |
| Step | Hyp | Ref | Expression |
|---|---|---|---|
| 1 | pm4.65 | |- ( -. ( -. ph -> ps ) <-> ( -. ph /\ -. ps ) ) |
|
| 2 | pm4.64 | |- ( ( -. ph -> ps ) <-> ( ph \/ ps ) ) |
|
| 3 | 1 2 | xchnxbi | |- ( -. ( ph \/ ps ) <-> ( -. ph /\ -. ps ) ) |