The minimum axial compressive load, $P$, required to initiate buckling for a pinned-pinned slender column with bending stiffness $EI$ and length $L$ is

1. $P=\dfrac{\pi^2 EI}{4L^2} \\$
2. $P=\dfrac{\pi^2 EI}{L^2} \\$
3. $P=\dfrac{3\pi^2 EI}{4L^2} \\$
4. $P=\dfrac{4\pi^2 EI}{L^2}$