It also can be of a function :)

来源: 于 2015-03-06 19:53:55 [旧帖] [给我悄悄话] 本文已被阅读：次

Ex: the characteristic function of any real-valued random variable is non-negative definite.

Ex: sinc(x) is non-negative definite.

Ex: -sinc(x) is non-positive definite.