Descent
From AoPSWiki
Given a permutation 
of 
, 
is said to have a descent at position 
if and only if 
. For example, the permutation 
has descents at positions 2 (since 
) and 4 (since 
).
The set of descents of a permutation is called its descent set. If is a permutation of then its descent set is some subset of 
.
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