1. Busemann, H.: The geometry of geodesics. Academic Press, New York (1955)
2. Busemann, H., Kelly, P.J.: Projective geometries and projective metrics. Academic Press, New York (1953)
3. Euler, L.: Geometrica et sphaerica quaedam, Memoires de lAcademie des Sciences de Saint-Petersbourg, 5 (1815), 96–114; Opera Omnia Series 1, vol. XXVI, 344–358; Original:
http://eulerarchive.maa.org/docs/originals/E749.pdf
; English translation:
http://eulerarchive.maa.org/Estudies/E749t.pdf
. Accessed 24 Sept 2018
4. Grünbaum, B., Klamkin, M.S.: Euler’s Ratio-Sum Theorem and Generalizations, Mathematics Magazine, 79:2, 122–130, (2006)
http://www.jstor.org/stable/27642919
. Accessed 24 Sept 2018
5. Grünbaum, B.: Cyclic ratio sums and products, Crux Mathematicorum, 24(1), 20–25, (1998)
https://cms.math.ca/crux/v24/n1/page20-25.pdf
. Accessed 24 Sept 2018