is every covariance matrix positive definite? is every covariance matrix positive is every covariance matrix positive is every covariance matrix definite? are all covariances positive is every covariance matrix positive is covariance positive is every covariance matrix positive or negative? is every covariance positive is every covariance matrix positive is every covariance matrix positive is every covariance matrix positively definite? is every covariance matrix positive definite definite positive covariance matrix is every covariance matrix positive definite? which matrix is positive is every covariance matrix positive is every covariance matrix positive definite is every covariance matrix positive? is every covariance matrix positive