For large n and m, but with n − m relatively small, these are the classical frequencies by Bohr 's correspondence principle E n − − E m ≈ ≈ h ( n − − m ) / T . In the formula above, T is the classical period of either orbit n or orbit m, since the difference between them is higher order in h. But for n and m small, or if n − m is large, the frequencies are not integer multiples of any single frequency.