Beyond Conway's concyclicity theorem: generalization and alternatives

Abstract

The famous concyclicity theorem stated by John H. Conway is here reconsidered by means of a parametrisation of the associated triangular configuration with arbitrary triplets of real numbers ($\alpha$;$\beta$;$\gamma$). This theorem, thus corresponding to the case ($\alpha$;$\beta$;$\gamma$)=(1;1;1), is generalized while demonstrating that there always exist an infinite family of such triplets which keeps unchanged the conclusion. The "anti-Conway" configuration corresponding to the case ($\alpha$;$\beta$;$\gamma$)=(-1;-1;-1) is also investigated : Xavier Dussau's theorem of concurrent lines is redemonstrated and completed by another concyclicity theorem. It is also proved that there exist in general a unique triplet ($\alpha$;$\beta$;$\gamma$)$\ne$(-1;-1;-1) which is a function of the sides of the considered triangle and which keeps unchanged the conclusion of Dussau's theorem.