σ ( n ) ≃ σ ˜  ( n ) = Q +  × A + B n          ( 42  )    MathType@MTEF@5@5@+=feaafiart1ev1aaatCvAUfKttLearuWrP9MDH5MBPbIqV92AaeXatLxBI9gBaebbnrfifHhDYfgasaacH8akY=wiFfYdH8Gipec8Eeeu0xXdbba9frFj0=OqFfea0dXdd9vqai=hGuQ8kuc9pgc9s8qqaq=dirpe0xb9q8qiLsFr0=vr0=vr0dc8meaabaqaciaacaGaaeqabaqabeGadaaakeaaiiGacqWFdpWCcqGGOaakcqWGUbGBcqGGPaqkcqWIdjYocuWFdpWCgaacaiabcIcaOiabd6gaUjabcMcaPiabg2da9iabdgfarnaaCaaaleqabaGaey4kaScaaOGaey41aq7aaOaaaeaacqWGbbqqcqGHRaWkdaWcaaqaaiabdkeacbqaaiabd6gaUbaaaSqabaGccaWLjaGaaCzcamaabmaabaGaeGinaqJaeGOmaidacaGLOaGaayzkaaaaaa@464C@