Mathematics/Geometry

Foundation of geometry in planes, and some complements: Excluding the parallel axioms
by Fumiya Iwama

Schutz' Independent Axioms for Minkowski Spacetime
by Richard Schmoetten, Jake Palmer and Jacques Fleuriot

Grothendieck's Schemes in Algebraic Geometry
by Anthony Bordg, Lawrence C. Paulson and Wenda Li

Tarski's Parallel Postulate implies the 5th Postulate of Euclid, the Postulate of Playfair and the original Parallel Postulate of Euclid
by Roland Coghetto

Poincaré Disc Model
by Danijela Simić, Filip Marić and Pierre Boutry

Complex Geometry
by Filip Marić and Danijela Simić

Hilbert's Nullstellensatz
by Alexander Maletzky

Octonions
by Angeliki Koutsoukou-Argyraki

Quaternions
by Lawrence C. Paulson

Projective Geometry
by Anthony Bordg

Gromov Hyperbolicity
by Sebastien Gouezel

Stewart's Theorem and Apollonius' Theorem
by Lukas Bulwahn

Minkowski's Theorem
by Manuel Eberl

Buffon's Needle Problem
by Manuel Eberl

Intersecting Chords Theorem
by Lukas Bulwahn

Ptolemy's Theorem
by Lukas Bulwahn

Chamber Complexes, Coxeter Systems, and Buildings
by Jeremy Sylvestre

Basic Geometric Properties of Triangles
by Manuel Eberl

The independence of Tarski's Euclidean axiom
by T. J. M. Makarios

Proving the Impossibility of Trisecting an Angle and Doubling the Cube
by Ralph Romanos and Lawrence C. Paulson

The General Triangle Is Unique
by Joachim Breitner