Sources page biographical material

6.AL. STEINER LEHMUS THEOREM
This has such an extensive history that I will give only a few items.
C. L. Lehmus first posed the problem to Jacob Steiner in 1840.

Rougevin published the first proof in 1842. ??NYS.

Jacob Steiner. Elementare Lösung einer Aufgabe über das ebene und sphärische Dreieck. J. reine angew. Math. 28 (1844) 375 379 & Tafel III. Says Lehmus sent it to him in 1840 asking for a purely geometric proof. Here he gives proofs for the plane and the sphere and also considers external bisectors.

Theodor Lange. Nachtrag zu dem Aufsatze in Thl. XIII, Nr. XXXIII. Archiv der Math. und Physik 15 (1850) 221 226. Discusses the problem and gives a solution by Steiner and two by C. L. Lehmus. Steiner also considers the external bisectors.

N. J. Chignell. Note 1031: A difficult converse. MG 16 (No. 219) (Jul 1932) 200-202. [The author's name is omitted in the article but appears on the cover.] 'Three fairly simple proofs', due to: M. J. Newell; J. Travers, improving J. H. Doughty, based on material in Lady's and Gentleman's Diary (1859) 87-88 & (1860) 84-86; Wm. Mason, found by Doughty, in Lady's and Gentleman's Diary (1860) 86.

H. S. M. Coxeter. Introduction to Geometry. Wiley, 1961. Section 1.5, ex. 4, p. 16. An easy proof is posed as a problem with adequate hints in four lines.

M. Gardner. SA (Apr 1961) = New MD, chap. 17. Review of Coxeter's book, saying his brief proof came as a pleasant shock.

G. Gilbert & D. MacDonnell. The Steiner Lehmus theorem. AMM 70 (1963) 79 80. This is the best of the proofs sent to Gardner in response to his review of Coxeter. A later source says this turned out to be identical to Lehmus' original proof!

Léo Sauvé. The Steiner Lehmus theorem. CM 2:2 (Feb 1976) 19 24. Discusses history and gives 22 references, some of which refer to 60 proofs.

Charles W. Trigg. A bibliography of the Steiner Lehmus theorem. CM 2:9 (Nov 1976) 191 193. 36 references beyond Sauvé's.

David C. Kay. Nearly the last comment on the Steiner Lehmus theorem. CM 3:6 (1977) 148 149. Observes that a version of the proof works in all three classical geometries at once and gives its history.