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I am a little unsure what you are asking. You seem to think that two Majorana fermions can interact in such a way that one gets to Dirac fermions. The Majorana fermion is its own antiparticle. The charge conjugation of $\psi$ is $C\psi~=~i\psi^*$. The appearance of $\psi$ and $C\psi$ in the Lagrangian means that the Majorana field must be electrically ...
A topological invariants is a continuous map $n$: $$\mathfrak{H}\ni H\mapsto n\left(H\right)\in S$$ where $H$ is the Hamiltonian of your system and $S$ is some topological space. $\mathfrak{H}$ is the space of all admissible Hamiltonians. Unfortunately the exact definition of $\mathfrak{H}$ is still a matter of current research. To keep things short, we ...