On the number of minimal 1-Steiner trees.
B. Aronov,
M. Bern,
and D. Eppstein.
Disc. & Comp. Geom. 12: 29–34, 1994,
doi:10.1007/BF02574363.
Given a \(d\)-dimensional set of \(n\) points, the number of combinatorially different minimum spanning trees that can be formed by adding one more point is within a polylogarithmic factor of \(n^d\).