Hjelmslev's theorem

== English == === Etymology === Named after Johannes Hjelmslev. === Proper noun === Hjelmslev's theorem (geometry) a theorem stating that if a set of points P i {\displaystyle {P_{i}}} lying on a line is isometrically mapped to points P i ′ {\displaystyle {P_{i}'}} on a different line, then the midpoints of the segments P i P i ′ {\displaystyle P_{i}P_{i}'} also lie on a line. ==== Translations ====