here
– – – – –
The parallel lines in Lobachevsky's geometry intersect.
– – – – –
They do not cross. In Lobachevsky, a straight may have several parallel lines, intersecting with each other, but not in contact with it itself. There will no longer be parallel crossings. (B & A B & A C!=> B & B C)

The joke:

A man runs through a psychiatric hospital and screams:
I will differentiate you!! I will integrate you!!! to
All of him, of course, are very afraid, run away with horror, hide. And he is happy and shouts his threats even louder and more terrifying. But then he notices in the corner a patient who sits and seems not to notice what is happening at all. “Now I am him,” thinks the first, climbs to him, makes terrible eyes and says:
I will differentiate you!! I will integrate you!! to
–...
I will differentiate you!! I will integrate you!! to
–...
I’m going to differentiate you... to integrate?
- And I "e^h"...
And I’ll tell you "y" the difference!! to