By rephrasing quantifier-free axioms as rules of derivation in sequent calculus, we show that the generalized Steiner–Lehmus theorem admits a direct proof in classical logic. This provides a partial answer to a question raised by Sylvester in 1852. We also present some comments on possible intuitionistic approaches.
"Negation-Free and Contradiction-Free Proof of the Steiner–Lehmus Theorem." Notre Dame J. Formal Logic 59 (1) 75 - 90, 2018. https://doi.org/10.1215/00294527-2017-0019