Nodal Quantum Numbers for Two-Electron States in Solids

The problem of construction and physical labelling of singlet and triplet zero total momentum two-electron states in solids is considered. It is shown that the wavefunctions belonging to repeating multi-dimensional irreducible representations can be labelled making use of theorem of induction transitivity. The intermediate group in this classification can be chosen depending on experimental nodal structure of superconducting order parameter. The application of the results to unconventional superconductors is discussed.