Lambert quadrangle

A quadrangle with right angles at three of the vertices. It was considered by J.H. Lambert (1766) in attempts to prove Euclid's parallelism postulate (cf. Fifth postulate). Of the three possible assumptions about the size of the fourth angle, that it is a right, an obtuse or an acute angle, the first is equivalent to Euclid's postulate and the second leads to a contradiction with the other axioms and postulates of Euclid. As for the third, Lambert conjectured that it is satisfied on an imaginary sphere.

References

[1]	V.F. Kagan, "Foundations of geometry" , 1 , Moscow-Leningrad (1949) (In Russian)
[2]	A.V. Pogorelov, "Lectures on the foundations of geometry" , Noordhoff (1966) (Translated from Russian)

References

[a1]	H. Busemann, P.J. Kelly, "Projective geometry and projective metrics" , Acad. Press (1953)
[a2]	H.S.M. Coxeter, "Introduction to geometry" , Wiley (1963)
[a3]	M. Greenberg, "Euclidean and non-Euclidean geometries" , Freeman (1974)
[a4]	N.W. [N.V. Efimov] Efimow, "Höhere Geometrie" , Deutsch. Verlag Wissenschaft. (1960) (Translated from Russian)
[a5]	A.P. Norden, "Elementare Einführung in die Lobatschewskische Geometrie" , Deutsch. Verlag Wissenschaft. (1958) (Translated from Russian)
[a6]	R. Bonola, "Non-Euclidean geometry" , Dover, reprint (1955) (Translated from Italian)