Connecting a Set of Circles with Minimum Sum of Radii
  We consider the problem of assigning radii to a given set of points in the plane, such that the resulting set of circles is connected, and the sum of radii is minimized. We show that the problem is polynomially solvable if a connectivity tree is given. If the connectivity tree is unknown, the problem is NP-hard if there are upper bounds on the radii and open otherwise. We give approximation guarantees for a variety of polynomial-time algorithms, describe upper and lower bounds (which are matching in some of the cases), provide polynomial-time approximation schemes, and conclude with experimental results and open problems. 
Erin Wolf Chambers, S\'andor P. Fekete, Hella-Franziska Hoffmann,   Dimitri Marinakis, Joseph S.B. Mitchell, Venkatesh Srini...
cs.CG cs.DS