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The new mathematical formalism for the Green's functions of interacting electrons in crystals is constructed. It is based on the theory of Green's functions and permutation groups. We constructed a new object of permutation groups, which we call double permutation (DP). DP allows one to take into consideration the symmetry of the ground state as well as energy and momentum conservation in every virtual interaction. We developed the classification of double permutations and proved the theorem, which allows the selection of classes of associated double permutations (ADP). The Green's functions are constructed for series of ADP. We separate in the DP the convolving columns by replacing the initial interaction between the particles with the effective interaction. In convoluting the series for Green's functions, we use the methods developed for permutation groups schemes of Young-Yamanuti. § ©

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Double permutation (DP) method is developed here for designing Feynman diagrams for mass operator (MO) of interacting electrons in high orders of perturbation theory (PT). The derived expression allows finding the Young diagrams for the class of permutations corresponding to disconnect Feynman diagrams. The classification of DPs, carried out before, allows to identify the permutations corresponding to disconnected, singly connected (improper) diagrams and to derive expressions for intolerant cycles of permutations. Ordering the nonprimed digits in natural order in the cycles of DP, we avoid the permutations, corresponding to the Feynman diagrams of the same topology because of other numbering of nodes. Thus, the numbers of considered permutations is sufficiently reduced: (from 24 to 6 and from 720 to 42) in the second and the third orders of PT. All 414 expressions (diagrams) for MO in the fourth order of PT were designed using this method. The use of group theory allows us to conclude that no more Feynman diagrams can be designed. The developed method can be used as algorithm for Feynman diagrams designing for MO of interacting electrons (one sort fermions) in higher orders of PT.