**Carl Theodore Heisel** (1852–1937)^{
[1]} was a mathematical
crank who wrote several books in the 1930s challenging accepted mathematical truths. Among his claims is that he found a way to
square the circle. He is credited with 24 works in 62 publications.^{
[2]} Heisel did not charge money for his books; he gave thousands of them away for free. Because of this, they are available at many libraries and universities. Heisel's books have historic and monetary value.^{
[3]}
Paul Halmos referred to one of Heisel's works as a "classic crank book."^{
[4]}

- Heisel, Carl Theodore (1934).
*Mathematical and geometrical demonstrations, disproving numerous theorems, problems, postulates, corolleries, axioms naturally growing out of the extraordinary discoveries of a lacking link*(2d. ed.). Cleveland.

**^**"Carl Theodore Heisel (1852–1937)".*Find a Grave*. Retrieved 31 May 2019.**^**"WorldCat Listing". Retrieved 16 May 2019.**^**Simanek, Donald E. "Carl Theodore Heisel Squares the Circle".*lockhaven.edu*. Retrieved 23 May 2019.**^**Paul R. Halmos (1970). "How to Write Mathematics".*L'Enseignement mathématique*.**16**(2): 123–152. – How to Write Mathematics