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An energy gap in nuclear matter is studied. The nucleon-nucleon potentials with strong repulsive part are used, which can reproduce the experimental phase shifts of the high energy nucleon-nucleon scattering up to 310 Mev. Solving the non-linear equation for the energy gap directly in the momentum space, we can obtain the following results.(i) An energy gap exists in relatively low density and for rather large effective mass (0.

The orthogonality condition model is applied to 2a+t system, and the structure of 11 B nucleus is investigated. The level energies, wave functions and electromagnetic properties are calculated. The existence of the states with well-developed molecule-like structure is suggested. The low-lying negative parity states are proved to have compact shell-like structure. The positive parity states are shown to have the structure of transient character between shell-like and molecule-like ones. The calculated energies of the low-lying negative parity states are somewhat lower than the experimental ones. The exchange effect among the three clusters is discussed, and considered to compensate the overbinding of the calculated energies of the low-lying negative parity states. The calculation for the low-lying negative parity states is further carried out including the coupling with the shell model configurations of lower symmetry; then, the energy spectrum is remarkably improved.

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