Extending our earlier study of nonlinear Bogolyubov-Valatin transformations (canonical transformations for fermions) for one fermionic mode, in the present paper we perform a thorough study of general (nonlinear) canonical transformations for two fermionic modes. We find that the Bogolyubov-Valatin group for n = 2 fermionic modes which can be implemented by means of unitary SU (2 n = 4) transformations is isomorphic to SO(6; R)/Z 2 . The investigation touches on a number of subjects. As a novelty from a mathematical point of view, we study the structure of nonlinear basis transformations in a Clifford algebra [specifically, in the Clifford algebra C(0, 4)] entailing (supersymmetric) transformations among multivectors of different grades. A prominent algebraic role in this context is being played by biparavectors (linear combinations of products of Dirac matrices, quadriquaternions, sedenions) and spin bivectors (antisymmetric complex matrices). The studied biparavectors are equivalent to Eddington's E-numbers and can be understood in terms of the tensor product of two commuting copies of the division algebra of quaternions H. From a physical point of view, we present a method to diagonalize any arbitrary two-fermion Hamiltonian. Relying on Jordan-Wigner transformations for two-spin-1 2 and single-spin-3 2 systems, we also study nonlinear spin transformations and the related problem of diagonalizing arbitrary two-spin-1 2 and single-spin-3 2 Hamiltonians. Finally, from a calculational point of view, we pay due attention to explicit parametrizations of SU (4) and SO(6; R) matrices (of respective sizes 4 × 4 and 6 × 6) and their mutual relation. † As a consequence of eq. (17) and the anti-Hermiticity of the operatorsĉ k , the operatorsĉ k [i.e., the basis elements of the paravector space associated with the Clifford algebra C(0, 5)] obey the relation (k, l = −1, . . . , 4)(304) 35 Below we have not indicated that the wedge product relates to a different vector space than in eq. (301), i.e.:
