“…The sets Φ + 0 , Φ + 1 are given as follows: In this subsection, we will construct composition series of standard Whittaker modules of N (ζ) explicitly by finding their Whittaker vectors. Similar computation was also given in [BCW,Section 5.1], where the authors concluded that all standard Whittaker modules are simple. However, by [CM,Theorem 6.7] and [MS1,Proposition 2.1(3)] there exist reducible standard Whittaker modules; see Section 5.3.3 for more details.…”