Energy Spectrum Symmetry of Heisenberg Model in Fock Space

We extend the BCS paring model with equally spaced energy levels to a general one-dimensional spin-1/2 Heisenberg model. The two well-known symmetries of the Heisenberg model, i.e. permutational and spin-inversion symmetries, no longer exist. However, when jointing these two operations together, we find a new symmetry of energy spectrum between its subspace n and subspace L-n of the Fock space. A rigorous proof is presented.