- #1
A_B
- 93
- 1
trying to prove dual of "there are at least tree points on every line"
Hi,
Assuming the propositions of incidence:
(1) on any two distinct points is at least one line.
(2) on any two distinct points is at most one line.
(3) on any two distinct lines is at least one point.
and the extensions
(4) not all points are on the same line.
(5) There are at least three points on every line.
I am asked to prove the dual of (5), that is, "there are at least 3 lines on every point."
(The duals of all other statements above have already been proven.)
Perhaps there is something I'm missing (and that's the reason for posting) but it seems to me that if only one point exists (call it P) and no lines then (1), (2), (3), (4) and (5) are all vacuously true. So this would present a counterexample to the sought-after theorem.
thanks!
A_B
Hi,
Assuming the propositions of incidence:
(1) on any two distinct points is at least one line.
(2) on any two distinct points is at most one line.
(3) on any two distinct lines is at least one point.
and the extensions
(4) not all points are on the same line.
(5) There are at least three points on every line.
I am asked to prove the dual of (5), that is, "there are at least 3 lines on every point."
(The duals of all other statements above have already been proven.)
Perhaps there is something I'm missing (and that's the reason for posting) but it seems to me that if only one point exists (call it P) and no lines then (1), (2), (3), (4) and (5) are all vacuously true. So this would present a counterexample to the sought-after theorem.
thanks!
A_B