AnalysisFunctionalAnalysis
A function is called a positive-definite (p.d.) kernel if and is a positive semi-definite (p.s.d.) kernel if equivalently, is a symmetric positive-semi definite (SPSD) kernel if the Gram Matrix is symmetric and positive-definite so that either
Eigenvalues of are non-negative
In applications, can be thought of as a similarity measure on .