Knowing that represents the critical stress of the pressure column, which of the following statements is correct? (1) The value of of the slender column is related to the material. (2) The value of of the medium-length column is independent of the material. (3) The value of for all types of columns is related to the slenderness ratio . (4) Assume the real stress of the column is less than , then the column will not be unstable.