if i=1,j=1∑naikijaj≥0∨a⊂R then k is positive definite.
X∗Y⊂x∗y this is a set.
if kn−>k then x−>+∞limaTkna≥0 =aTKa≥0 KK=>Kn−1k=kn they all are P.S.D,then all ≥0
Thy: Let X be a nonempty set x0⊂X and Let ϕ:X∗X−>IR be a symmetric kernel. P−atK(x,y)=ϕ(x,x0)+ϕ(y,x0)−ϕ(x,y)=ϕ(x0,y0) then k is P.D iff ϕ is negative definite.