4. A process according to claim 3, wherein: the learning phase further comprises the step of storing the covariance matrix X in the dictionary; the test phase calculates the inverse matrix Y.sup.-1 ; and the test phase directly calculates the trace of a product of two symmetrical matrices A and B according to the equation: ##EQU16##
5. A process according to claim 4 wherein the determining step uses the function: EQU traceprod(Y,X.sup.-1).multidot.traceprod(X,Y.sup.-1)=p.sup.2 a/h.