  Group theoretical technique for construction of two-electron wavefunctions
with zero total momentum in solids based on the Mackey theorem on symmetrized
squares is developed. These states correspond to singlet and triplet Cooper
pairs. The nodal structure of these states is investigated group theoretically
and compared with experimental data for unconventional superconductors. It is
shown that when the Mackey theorem is applied twice, the main four-electron
states in solids can be constructed. Possible connection of these states with
experimental data is discussed.
