Constructible number
We say that a nonnegative real number is constructible if a segment of length
can be constructed with a straight edge and compass starting with a segment of length
.
We say that a complex number is constructible if
and
are both constructible (we also say that the point
is constructible). It is easy to show that
is constructible iff the point
can be constructed with a straight edge and compass in the cartesian plane starting with the points
and
.