Euler's [nine point circle][1] was [never discussed][2] by Euler. This is an error of the "argument by authority" type: Catalan propagated that incorrect attribution made by another scholar, the "learned Terquem", without checking it himself.


  [1]: https://en.wikipedia.org/wiki/Nine-point_circle
  [2]: https://web.archive.org/web/20080606002016/http://sunsite.utk.edu/math_archives/.http/hypermail/historia/jun00/0134.html