OCLC, Oct. 30, 2002 (hdgs.: Otto, Max Carl, 1876- ; usage: Max Carl Otto, Max C. Otto, M.C. Otto)
Havard Square Library website, viewed March 16, 2018 (Otto, Max (1876-1968); Professor Max Carl Otto was born in the historic town of Zwickau, Saxony, in 1876; he attended Carroll College in Waukesha, and the University of Wisconsin in Madison, where he majored in history under the great Frederick Jackson Turner; received a PhD in philosophy in 1911; taught philosophy at the University of Wisconsin) http://www.harvardsquarelibrary.org/biographies/max-otto