Formula: ||f||_H^2 = sum_i sum_j beta_i beta_j K(x_i, x_j) = beta' K beta
References
Montesinos Lopez, Montesinos Lopez and Crossa (2022), Multivariate Statistical Machine Learning Methods for Genomic Prediction, Springer, doi:10.1007/978-3-030-89010-0. Chapter 8, Eq. (8.2) p. 254: the squared norm of f in the reproducing kernel Hilbert space is beta'K beta. Read from the chapter PDF, not recalled.
