— Function: int gsl_matrix_isnull (const gsl_matrix * m)
— Function: int gsl_matrix_ispos (const gsl_matrix * m)
— Function: int gsl_matrix_isneg (const gsl_matrix * m)
— Function: int gsl_matrix_isnonneg (const gsl_matrix * m)

These functions return 1 if all the elements of the matrix m are zero, strictly positive, strictly negative, or non-negative respectively, and 0 otherwise. To test whether a matrix is positive-definite, use the Cholesky decomposition (see Cholesky Decomposition).