Fellow Puny Humans,
A geometric net is a system of points and lines that obeys three axioms:
- Each line is a set of points.
- Distinct line has at most one point in common.
- If $p$ is a point and $L$ is a line with $p \notin L$, then there is exactly one line $M$ such that $p \in M$ and $L \cap M = \phi $.
And whenever $L \cap M = \phi$ we say that $L$ is parallel to $M$ i.e $L || M$.
So far so good.
I want to partition these lines of geometric net into equivalence classes with two lines in same class if they are equal or parallel. One can easily show that binary operation equal or parallel is an equivalence relation.
Let's say there are $m$ such classes, then how many points does a line have in each class? For a given line $l$ in any class, if a point $p \in l$ then how many lines passes through $p$.
For example, if I partition them into two classes $CL_1$ and $CL_2$ of parallel or equal lines, then number of points on any line in $CL_1$ is equal to number of lines in $CL_2$. This implies that each point belongs to two line. Can this be extended to a case when number of classes are $m$ i.e. each point belong to $m$ lines? I am confused because I can not show it for the case when more than two lines passes through the same point.
This problem is from TAOCP 4(a) : combinatorial searching Problem 21. (Edision Wesly).