∇ F +  ( E ) ≃ p N obs     ( 1 − p )  ℓ h  − N obs      β ( N obs   , ℓ h  − N obs   + 1 )   × G       ( 21  )    MathType@MTEF@5@5@+=feaafiart1ev1aaatCvAUfKttLearuWrP9MDH5MBPbIqV92AaeXatLxBI9gBaebbnrfifHhDYfgasaacH8akY=wiFfYdH8Gipec8Eeeu0xXdbba9frFj0=OqFfea0dXdd9vqai=hGuQ8kuc9pgc9s8qqaq=dirpe0xb9q8qiLsFr0=vr0=vr0dc8meaabaqaciaacaGaaeqabaqabeGadaaakeaacqGHhis0cqWGgbGrdaahaaWcbeqaaiabgUcaRaaakiabcIcaOGqabiab=veafjabcMcaPiabloKi7maalaaabaGaemiCaa3aaWbaaSqabeaacqWGobGtdaWgaaadbaGaee4Ba8MaeeOyaiMaee4CamhabeaaaaGccqGGOaakcqaIXaqmcqGHsislcqWGWbaCcqGGPaqkdaahaaWcbeqaaiabloriSnaaBaaameaacqWGObaAaeqaaSGaeyOeI0IaemOta40aaSbaaWqaaiabb+gaVjabbkgaIjabbohaZbqabaaaaaGcbaacciGae4NSdiMaeiikaGIaemOta40aaSbaaSqaaiabb+gaVjabbkgaIjabbohaZbqabaGccqGGSaalcqWItecBdaWgaaWcbaGaemiAaGgabeaakiabgkHiTiabd6eaonaaBaaaleaacqqGVbWBcqqGIbGycqqGZbWCaeqaaOGaey4kaSIaeGymaeJaeiykaKcaaiabgEna0kab=DeahjaaxMaacaWLjaWaaeWaaeaacqaIYaGmcqaIXaqmaiaawIcacaGLPaaaaaa@6649@