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==Convex polygons==
==Tessellation by Schwarz triangles==
In this section tessellations of the hyperbolic upper half plane by Schwarz triangles will be discussed using elementary methods. For triangles withwithout ideal"cusps"—angles vertices,equal i.e.to wherezero allor threeequivalently anglesvertices areon strictlythe positive,real theaxis—the elementary approach of {{harvtxt|Caratheodory|1954}} will be followed. For triangles with one or two ideal vertices, i.e. one or two angles equal to zerocusps, elementary arguments of {{harvtxt|Evans|1973}}, simplifying the approach of {{harvtxt|Hecke|1935}}, will be used: in the case of a Schwarz triangle with one angle zero and another a right angle, the orientation-preserving subgroup of the reflection group of the triangle is a [[Hecke group]]. For an ideal triangle in which all angles are zero, so that all vertices lie on the real axis, the existence of the tessellation will be established by relating it to the [[Farey series]] described in {{harvtxt|Hardy|Wright|1979}} and {{harvtxt|Series|2015}}. In this case the tessellation can be considered as that associated with three touching circles on the [[Riemann sphere]], a limiting case of configurations associated with three disjoint non-nested circles and their reflection groups, the so-called "[[Schottky group]]s", described in detail in {{harvtxt|Mumford|Series|Wright|2015}}. Alternatively—by dividing the ideal triangle into six triangles with angles 0, {{pi}}/2 and {{pi}}/3—the tessellation by ideal triangles can be understood in terms of tessellations by triangles with one or two ideal vertices.
===Triangles withwithout no ideal verticescusps===
Suppose that the [[hyperbolic triangle]] Δ has angles {{pi}}/''a'', {{pi}}/''b'' and {{pi}}/''c'' with ''a'', ''b'', ''c'' integers greater than 1. The hyperbolic area of Δ equals {{pi}} – {{pi}}/''a'' – {{pi}}/''b'' – {{pi}}/''c'', so that
The equality above implies that if one of the angles is a right angle, say ''a'' = 2, then both ''b'' and ''c'' are greater than 2 and one of them, ''b'' say, must be greater than 3. In this case, reflecting the triangle across the side AB gives an isosceles hyperbolic triangle with angles {{pi}}/''c'', {{pi}}/''c'' and 2{{pi}}/''b''. The construction of the tessellation that is given below applies equally well in this case because the doubled triangle is isosceles.<ref>{{harvnb|Caratheodory|1954|pages=181–182}}</ref>
===Triangles with one or two ideal verticescusps===
===Ideal triangles===
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