On approximations involved in the theory of positron transport in gases in electric and magnetic fieldsA. Banković, S. Dujko, R.D. White, S.J. Buckman and Z.Lj. PetrovićEur. Phys. J. D, 66 7 (2012) 174DOI: https://doi.org/10.1140/epjd/e2012-20696-1