Wilhelm Ahrens was a great German exponent of recreational mathematics whose Mathematische Unterhaltungen und Spiele¹ is one of the most scholarly of all books on the subject. Ahrens was born in Lübz on the river Elde in northeast Germany and studied from 1890 to 1897 in Rostock, where he graduated summa cum laude. From 1895 to 1896 he taught at a school in Antwerp and then studied under Sophus Lie in Leipzig. In 1897 he was a teacher in Magdeburg, from 1901 at the engineering school. Inspired by Lie, he wrote: "On transformation groups, all of whose subgroups are invariant" (Hamburger Math Society Vol 4, 1902).

 

Reference

1. Ahrens, W. Mathematische Unterhaltungen und Spiele. Leipzig, Germany: Teubner, 1910.