A long solenoid of length \(L\) and radius \(r_1\) having \(N_1\) turns is surrounded symmetrically by a coil of radius \(r_2\,(>r_1)\) having \(N_2\) turns \((N_2 \ll N_1)\) around its mid-point. Derive an expression for the mutual inductance of the solenoid and the coil. Is \(M_{12}=M_{21}\) valid in this case?