The Weighted Degree String kernel.

The WD kernel of order d compares two sequences X and
Y of length L by summing all contributions of k-mer matches of
lengths k in 1...d , weighted by coefficients beta_k. It
is defined as

    k(X, Y)=\sum_{k=1}^d\beta_k\sum_{l=1}^{L-k+1}I(u_{k,l}(X)=u_{k,l}(Y)).

Here, $u_{k,l}(X)$ is the string of length k starting at position
l of the sequence X and I(.) is the indicator function
which evaluates to 1 when its argument is true and to 0
otherwise.

