Bergman et al., 1979: https://www.ncbi.nlm.nih.gov/pubmed/443421

Defronzo et al., 1979: https://www.ncbi.nlm.nih.gov/pubmed/382871

Ekstrand et al., 1992: https://www.ncbi.nlm.nih.gov/pubmed/1549847

Matsuda et al., 1999: https://www.ncbi.nlm.nih.gov/pubmed/10480510

Katz et al., 2000: https://www.ncbi.nlm.nih.gov/pubmed/10902785

Python sci-kit code: http://scikit-learn.org/stable/modules/kernel_ridge.html