Hilbert's Error?

International audience; Geometric constructions have been a topic of interest among mathematicians for centuries. The Euclidean tradition uses two instruments: a straightedge (ruler) and a compass. However, limited constructions have also been studied. The Mohr–Mascheroni theorem says that every construction using both instruments can be replaced by a construction of the same object that uses only a compass. (Obviously, a line cannot be drawn with a compass; we construct two distinct points on that line instead.) Another result, the Poncelet–Steiner theorem, says that in a construction using straightedge and compass, a single application of a compass suffices: if a circle with its center is given, then every straightedge–compass construction can be performed with only a straightedge. Today, such results are considered to belong to recreational mathematics, and they can be found in many books, such as [6], for example.