The set of forbidden pattern is
 
                                  {[1, 2, 3]}
 
                         This is the (RIGOROUS) scheme
 
Warning, new definition for Chi
[{[2, 1], [1, 2]}, {[], [1]}, table([
 
    [2, 1] = 1
 
    [1, 2] = 2
 
    ]), table([
 
    [] = {[1]}
 
    [1] = {[2, 1], [1, 2]}
 
    ]), table([
 
    [2, 1] = {}
 
    [] = {}
 
    [1] = {}
 
    [1, 2] = {2}
 
    ])]
 
                              This is the sequence
 
[1, 1, 2, 5, 14, 42, 132, 429, 1430, 4862, 16796, 58786, 208012, 742900,
 
    2674440, 9694845, 35357670, 129644790, 477638700, 1767263190, 6564120420]
 
                    This is the (Semi-Rigorous) Recurrence
 
                              2 - 4 n + (1 + n) N