The Galois Complexity of Graph Drawing: Why Numerical Solutions are   Ubiquitous for Force-Directed, Spectral, and Circle Packing Drawings
  Many well-known graph drawing techniques, including force directed drawings, spectral graph layouts, multidimensional scaling, and circle packings, have algebraic formulations. However, practical methods for producing such drawings ubiquitously use iterative numerical approximations rather than constructing and then solving algebraic expressions representing their exact solutions. To explain this phenomenon, we use Galois theory to show that many variants of these problems have solutions that cannot be expressed by nested radicals or nested roots of low-degree polynomials. Hence, such solutions cannot be computed exactly even in extended computational models that include such operations. 
Michael J. Bannister, William E. Devanny, David Eppstein, Michael T.   Goodrich
cs.CG