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Resonant e + e − pair production by an electron in a magnetic field near the process threshold has been analytically studied. Using the Nikishov's theorem an estimation of the number of events has been made in the magnetic field equivalent to laser wave in the SLAC experiment [D. Burke et al., Phys. Rev. Lett. 79, 1626(1997]. The obtained estimation is in reasonable agreement with the experimental data. 13.88.+e

Spin and polarization effects and correlations between them in the processes of pair production by a photon and synchrotron radiation in a magnetic field are considered. Expressions for the probabilities of the processes with arbitrary polarizations of the particles are obtained. These expressions are analyzed in detail in both the Lowest Landau Levels and ultrarelativistic approximations. PACS numbers: 12.20.-mQuantum electrodynamics , 13.88.+ePolarization in interactions and scattering