<human>: Answer by taking a quote from the following article:

Friedrich Ludwig Gottlob Frege (; German: ['gotlo:p 'fre:g@]; 8 November 1848 - 26 July 1925) was a German philosopher, logician, and mathematician. He is understood by many to be the father of analytic philosophy, concentrating on the philosophy of language and mathematics. Though largely ignored during his lifetime, Giuseppe Peano (1858-1932) and Bertrand Russell (1872-1970) introduced his work to later generations of logicians and philosophers.

Frege matriculated at the University of Jena in the spring of 1869 as a citizen of the North German Confederation. In the four semesters of his studies he attended approximately twenty courses of lectures, most of them on mathematics and physics. His most important teacher was Ernst Karl Abbe (1840-1905; physicist, mathematician, and inventor). Abbe gave lectures on theory of gravity, galvanism and electrodynamics, complex analysis theory of functions of a complex variable, applications of physics, selected divisions of mechanics, and mechanics of solids. Abbe was more than a teacher to Frege: he was a trusted friend, and, as director of the optical manufacturer Carl Zeiss AG, he was in a position to advance Frege's career. After Frege's graduation, they came into closer correspondence.  His other notable university teachers were Christian Philipp Karl Snell (1806-86; subjects: use of infinitesimal analysis in geometry, analytical geometry of planes, analytical mechanics, optics, physical foundations of mechanics); Hermann Karl Julius Traugott Schaeffer (1824-1900; analytical geometry, applied physics, algebraic analysis, on the telegraph and other electronic machines); and the philosopher Kuno Fischer (1824-1907; Kantian and critical philosophy).  Starting in 1871, Frege continued his studies in Gottingen, the leading university in mathematics in German-speaking territories, where he attended the lectures of Rudolf Friedrich Alfred Clebsch (1833-72; analytical geometry), Ernst Christian Julius Schering (1824-97; function theory), Wilhelm Eduard Weber (1804-91; physical studies, applied physics), Eduard Riecke (1845-1915; theory of electricity), and Hermann Lotze (1817-81; philosophy of religion). Many of the philosophical doctrines of the mature Frege have parallels in Lotze; it has been the subject of scholarly debate whether or not there was a direct influence on Frege's views arising from his attending Lotze's lectures.  In 1873, Frege attained his doctorate under Ernst Christian Julius Schering, with a dissertation under the title of "Ueber eine geometrische Darstellung der imaginaren Gebilde in der Ebene" ("On a Geometrical Representation of Imaginary Forms in a Plane"), in which he aimed to solve such fundamental problems in geometry as the mathematical interpretation of projective geometry's infinitely distant (imaginary) points.  Frege married Margarete Katharina Sophia Anna Lieseberg (15 February 1856 - 25 June 1904) on 14 March 1887. CANNOTANSWER

What did he study at Jena?
<bot>: he attended approximately twenty courses of lectures, most of them on mathematics and physics.