  We consider a strictly isolated single-mode optical cavity resonating at
angular frequency omega containing atoms whose one-electron level energies are
supposed to be: hbar*omega, 2*hbar*omega,...B*hbar\omega, and m photons. If
initially the atoms are in their highest energy state and m=0, we find that at
equilibrium: variance(m)/mean(m)=(B+1)/6, indicating that the internal field
statistics is sub-Poissonian if the number of atomic levels B does not exceed
4. Remarkably, this result does not depend on the number of atoms, nor on the
number of electrons that each atom incorporates. Our result has application to
the statistics of the light emitted by pulsed lasers and nuclear magnetic
resonance. On the mathematical side, the result is based on the restricted
partitions of integers.
