The Distribution of Double Deficiencies in Pattern-Avoiding Permutations

Abstract: We study the distribution of the number of double deficiencies (DD) in permutations of length n avoiding one or two patterns of length 3. Using structural decompositions of these avoidance classes--together with a lattice-path decomposition in the 321-avoiding case--we derive functional equations and convolution-type recurrences that efficiently compute the corresponding double-deficiency generating functions in all but one single-pattern case. In the 321-avoiding permutations, the resulting generating function is algebraic; we derive exact formulas for the mean and variance and prove that the distribution is close in total variation to Bin(n-2,1/4), with an explicit convergence rate. We also identify a DD-preserving symmetry that yields DD-Wilf equivalences, reducing the number of two-pattern cases that need to be considered separately. For the resulting two-pattern classes, we obtain explicit recurrences, including C-finite relations.