Transformation of electron spectrum in the Hubbard model

A new functional representation for strongly correlated systems is used to study the evolution of the electron spectrum in the Hubbard model with increasing $U$. It is established that the parametric instability of the ground state of a strongly correlated metal with 1/4 filled band can be described by linear transformations of the dynamic fields. It is shown that the operator of the transformations has a nontrivial kernel. It is noted that the considered system can serve as a point of departure for interpreting structural transitions of $2k_F$ and $4k_F$ types in the quasione-dimensional metal MEM(TCNQ)$_2$.