Centering. SVMs tend to perform better if the feature vectors {φ(G 1 ), ..., φ(G N )} are first centered . Again to avoid explicit construction of them, the double-centering trick allows us to obtain the inner products of the centered feature vectors from the inner products of the uncentered feature vectors in the Gram matrix K. Particularly, the centered Gram matrix K := CKC with centering matrix C = I -1 N oo (I the identity matrix, o a vector of ones) .