In the  equation  $\left(\mathrm{P}+\frac{\mathrm{a}}{{\mathrm{v}}^2}\right)=\mathrm{b}\frac{\mathrm{\theta }}{\mathrm{v}}$ ,  P is pressure, V is volume and $\mathrm{\theta }$ is absolute temperature. If a and b are constants, the dimension formula of a will be