On Lobachevsky`s trigonometric formulae
... the family of non-intersecting lines was used by the three founders of hyperbolic geometry. 14In using the transitivity property, it is necessary to keep track of the direction of parallelism for pairs of parallel lines involved. ...
... the family of non-intersecting lines was used by the three founders of hyperbolic geometry. 14In using the transitivity property, it is necessary to keep track of the direction of parallelism for pairs of parallel lines involved. ...