the determinant of the matrix of which *this is the LU decomposition. It has only linear complexity (that is, O(n) where n is the dimension of the square matrix) as the LU decomposition has already been computed.
ei_assert(m_isInitialized && "LU is not initialized.");
ei_assert(m_lu.rows() == m_lu.cols() && "You can't take the determinant of a non-square matrix!");
return Scalar(m_det_pq) * Scalar(m_lu.diagonal().prod());