ARTICLE Ginsbourger_J.STAT.PLAN.INFERENCE_2016/IDIAP On degeneracy and invariances of random fields paths with applications in Gaussian process modelling Ginsbourger, David Roustant, Olivier Durrande, Nicolas Covariance kernels Linear operators RKHS Structural priors Journal of Statistical Planning and Inference 170 117-128 2016 10.1016/j.jspi.2015.10.002 doi