ðeɪ ɑːɹ ˈɛlɪmənts ʌvðə vˈækjuːm ɐfˈaɪn vˈɜːtɛks ˈældʒᵻbɹə æt kɹˈɪɾɪkəl lˈɛvəl , wˌɛɹ ɪz ðə dˈuːəl kˈɑːksɛɾɚ nˈʌmbɚ fɔːɹ wˌɪtʃ ɑːɹ ɐnˈaɪəlˌeɪɾᵻd baɪ ðə pˈɑːzᵻtˌɪv lˈuːp ˈældʒᵻbɹə pˈɑːɹt ʌv , ðæt ˈɪz,
They are elements  of the vacuum affine vertex algebra at critical level , where  is the dual Coxeter number for  which are annihilated by the positive loop algebra  part of , that is,