Two patterns 
and 
belong to the same Wilf class if 
for all 
, where 
denotes the set of permutations on 
that avoid the
pattern 
.
Two sets having the same Wilf class are said to be Wilf
equivalent.
Wilf Class
See also
Avoided Pattern, Wilf Equivalent, Permutation PatternExplore with Wolfram|Alpha
References
Mansour, T. "Permutations Avoiding a Pattern from
and at Least Two Patterns from 
." 31 Jul 2000. http://arxiv.org/abs/math.CO/0007194.Referenced on Wolfram|Alpha
Wilf ClassCite this as:
Weisstein, Eric W. "Wilf Class." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/WilfClass.html