A steepest descent calculation of RNA pseudoknots
  We enumerate possible topologies of pseudoknots in single-stranded RNA molecules. We use a steepest-descent approximation in the large N matrix field theory, and a Feynman diagram formalism to describe the resulting pseudoknot structure. 
M. Pillsbury (1), H. Orland (2), A. Zee (1 and 3) ((1) UC Santa   Barbara, (2) CEA Saclay, (3) Institute for Theoretical Phy...