Consider the following inductively defined set. Given a collection U of unit magnitude complex numbers, and a set initially containing just 0 and 1, through each point in the set, draw lines whose angles with the real axis are in U. Add every intersection of such lines to the set. Upon taking the closure, we obtain R(U). We investigate for which U the set R(U) is a ring. Our main result holds when 1 is in U and the cardinality of U is at least 4. If P is the set of real numbers in R(U) generated in the second step of the construction, then R(U) equals the module over Z[P] generated by the set of points made in the first step of the construction. This lets us show that whenever the pairwise products of points made in the first step remain inside R(U), it is closed under multiplication, and is thus a ring.
Bahr, Jackson and Roth, Arielle
"Subrings of C Generated by Angles,"
Rose-Hulman Undergraduate Mathematics Journal: Vol. 17:
1, Article 2.
Available at: https://scholar.rose-hulman.edu/rhumj/vol17/iss1/2