The half-life of a theorem, or Arnold's principle at work This was essentially known to H. S. A. Potter [Amer. Math. Monthly 57, (1950). 321–322] and H. Wielandt [Math. Nachr. 4, (1951). 308–314]. See the historical paper in Amer. Math. Monthly 111 (2004), no. 8, 655–667.