Editing Angle of parallelism (section)

==History==
{{Anchor|Lobachevsky originator}} <!-- Lobachevsky's formula links here -->

The '''angle of parallelism''' was developed in 1840 in the German publication "Geometrische Untersuchungen zur Theory der Parallellinien"  by [[Nikolai Lobachevsky]].

This publication became widely known in English after the Texas professor [[G. B. Halsted]] produced a translation in 1891. (''Geometrical Researches on the Theory of Parallels'')

The following passages define this pivotal concept in hyperbolic geometry:
:''The angle HAD between the parallel HA and the perpendicular AD is called the parallel angle (angle of parallelism) which we will here designate by Π(p) for AD = p''.<ref name=Lob>[[Nikolai Lobachevsky]] (1840) [[G. B. Halsted]] translator (1891) [https://archive.org/details/geometricalresea00loba Geometrical Researches on the Theory of Parallels]</ref>{{rp|13}}<ref>{{cite book|last1=Bonola|first1=Roberto|title=Non-Euclidean geometry : a critical and historical study of its developments|url=https://archive.org/details/noneuclideangeom0000bono|url-access=registration|date=1955|publisher=Dover|location=New York, NY|isbn=0-486-60027-0|edition=Unabridged and unaltered republ. of the 1. English translation 1912.}}</ref>