The n -by- n matrix M is symmetric, and it can be positive definite ( M > 0 ) or semi-positive definite ( M ≥ ≥ 0 ) . Typically, it is assumed that M is positive definite; however, it is not uncommon to derive the unconstrained equations of motion of the system S such that M is only semi-positive definite; i.e., the mass matrix may be singular (it has no inverse matrix).