With given permutation of $1,\ldots,n$ for example (in one-line notation): 3 5 1 2 4 6
.
How to find amount of ascending subsequences of length 3 in the second row of the permutation ?
There's $n!/k!(n-k)!$ of subsequences of length $k$ for the identity permutation 1 2 3 4 5 6
$\cdots$n
.
How to deal with this problem ?
Thanks.