Planar Approximation as Two-Field Boltzmann Theory
  A modified interaction representation for the master field describing connected $SU(N)$-invariant Wightman's functions in the large $N$ limit of matrix fields is constructed. This construction is based on the representation of the master field in terms of Boltzmannian field theory found before. In the modified interaction representation we deal with two scalar Boltzmann fields ({\it up} and {\it down} fields). For up and down fields only half-planar diagrams contribute and this could help to write down a recursive set of non-linear integral-differential equations summing up planar diagrams. 
I.Ya. Aref'eva