On approximations involved in the theory of positron transport in gases in electric and magnetic fields A. Banković, S. Dujko, R.D. White, S.J. Buckman and Z.Lj. Petrović Eur. Phys. J. D, 66 7 (2012) 174 Published online: 10 July 2012 DOI: 10.1140/epjd/e2012-20696-1