 Frege never fully recovered from the fatal flaw discovered in the foundations of his Grundgesetze.  His attempts at salvaging the work by restricting Basic Law V were not successful. However, he continued teaching at Jena, and from 1903–1917, he published six papers, including ‘What is a Function?’ (1904) and ‘On the Foundations of Geometry’ (Frege 1903b and 1906). In the latter, Frege criticized Hilbert’s understanding and use of the axiomatic method (see the entry on the   Frege-Hilbert controversy). From this time period, we have the lecture notes that Rudolf Carnap took as a student in two of his courses (see Reck and Awodey 2004). In 1917, he retired from the University of Jena.