Hjelmslev's theorem

Etymology
Named after.

Proper noun

 * 1)  a theorem stating that if a set of points $${P_i}$$ lying on a line is isometrically mapped to points $${P_i'}$$ on a different line, then the midpoints of the segments $$P_iP_i'$$ also lie on a line.

Translations

 * German: Satz von Hjelmslev