For stable structures, one of the important properties of flexibility and stiffness matrices is that the element on the main diagonal
(i) of a stiffness matrix must be negative.
(ii) of a stiffness matrix must be positive
(iii) of a flexibility matrix must be positive
(iv) of a flexibility matrix must be negative1
(i) and (iii)
2
(i) and (iv)
3
(ii) and (iii)
4
(ii) and (iv)