Nonnegative minors of minor matrices
  Using the relationship between totally nonnegative matrices and directed acyclic weighted planar networks, we show that $2\times 2$ minors of minor matrices of totally nonnegative matrices are also nonnegative. We give a combinatorial interpretation for the minors of minor matrices in terms of the weights of families of paths in a network. 
David A. Cardon and Pace P. Nielsen
math.CO math.RA