Description: Disjunction in terms of conjunction (De Morgan's law). Compare Theorem *4.57 of WhiteheadRussell p. 120. (Contributed by NM, 3-Jan-1993) (Proof shortened by Andrew Salmon, 7-May-2011)
| Ref | Expression | ||
|---|---|---|---|
| Assertion | oran | |- ( ( ph \/ ps ) <-> -. ( -. ph /\ -. ps ) ) |
| Step | Hyp | Ref | Expression |
|---|---|---|---|
| 1 | pm4.56 | |- ( ( -. ph /\ -. ps ) <-> -. ( ph \/ ps ) ) |
|
| 2 | 1 | con2bii | |- ( ( ph \/ ps ) <-> -. ( -. ph /\ -. ps ) ) |