He edited the letters exchanged between Carl Friedrich Gauss and Wolfgang Bolyai, made contributions to editions of the collected works of Euler and Gauss (for whose works he wrote Gauss als Geometer), and edited the Geometrischen Untersuchungen by Wolfgang and Johann Bolyai (published in 1913).
He compiled the correspondence between Carl Friedrich Gauss and Wolfgang Bolyai, made contributions to the publication of Euler's and Gauss' comprehensive works (for whose writings he wrote a biography), and edited the Geometric Researches by Wolfgang and Johann Bolyai (published in 1913).