Answer by Nicolast for Converse to Euclid's fifth postulate
It is a very nice question.I am under the impression that a Riemannian metric on $\mathbb R^2$ which has conjugate points cannot satisfy the fifth postulate.Now, there are various results showing that,...
View ArticleConverse to Euclid's fifth postulate
There is a fascinating open problem in Riemannian Geometry which I would like to advertise here because I do not think that it is as well-known as it deserves to be. Euclid's famous fifth postulate, or...
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