# Saccheri quadrangle

From Encyclopedia of Mathematics

2020 Mathematics Subject Classification: *Primary:* 51M10 *Secondary:* 01A50 [MSN][ZBL]

A quadrangle $ABCD$, right-angled at $A$ and $B$ and with equal sides $AD$ and $BC$. It was discussed by G. Saccheri (1733) in attempts to prove Euclid's fifth postulate about parallel lines. Of the three possibilities regarding the angles at $C$ and $D$: they are right angles, they are obtuse angles or they are acute angles, the first is equivalent to the fifth postulate, and the second leads to spherical or elliptic geometry. As regards the third possibility, Saccheri made the erroneous deduction that it also contradicts the other axioms and postulates of Euclid.

#### References

[1] | V.F. Kagan, "Foundations of geometry" , 1 , Moscow-Leningrad (1949) (In Russian) |

[2] | A.V. Pogorelov, "Foundations of geometry" , Noordhoff (1966) (Translated from Russian) |

[a1] | R. Bonola, "Non-Euclidean geometry" , Dover, reprint (1955) pp. 23 (Translated from Italian) |

[a2] | H.S.M. Coxeter, "Non-Euclidean geometry" , Univ. Toronto Press (1965) pp. 5, 190 |

[a3] | N.V. Efimov, "Higher geometry" , MIR (1980) (Translated from Russian) |

[a4] | K. Borsuk, W. Szmielew, "Foundations of geometry" , North-Holland (1960) |

**How to Cite This Entry:**

Saccheri quadrangle.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Saccheri_quadrangle&oldid=53799

This article was adapted from an original article by A.B. Ivanov (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article