Theodor Spieker (8 August 1823 – 9 April 1913) was a German mathematician, a teacher at a gymnasium in Potsdam.
[1] Spieker's geometry textbook Lehrbuch der ebenen Geometrie mit Übungs-Aufgaben für höhere Lehranstalten (Verlag von August Stein, Potsdam, 1862)[2][3] was republished in many editions.
A copy of this textbook was given to Albert Einstein by his tutor when Einstein was twelve, and quickly led Einstein to become interested in higher mathematics.
[4] Spieker is the namesake of the Spieker circle of a triangle (the circle inscribed in its medial triangle) and the Spieker center (the center of the Spieker circle).