The notations that we use are summarized below. We followed
the notation used in Szedmak and Shawe-Taylor (2005). Note that
we assume every Hilbert space mentioned has finite dimension
and is defined above the real numbers.
Hn is an arbitrary Hilbert space with dimension n.
Hx is a Hilbert space comprising the possible input vectors.
H/(x) is a Hilbert space comprising the feature vectors.
Hy is a Hilbert space comprising the label vectors.