${\mathrm{E}}_{{\mathcal{D}}_t}[{\mathit{tf}}_d]=l_t/|{\mathcal{D}}_t|$	average frequency of term t in the documents in which the term occurs
${\mathrm{E}}_{{\mathcal{T}}_d}[{\mathit{tf}}_d]=l_d/|{\mathcal{T}}_d|$	average term frequency of terms that occur in document d
${\overline{l}}_d:={\mathrm{E}}_{\mathcal{D}}[l_d]=l_c/|\mathcal{D}|$	average document length
${\overline{l}}_t:={\mathrm{E}}_{\mathcal{T}}[l_t]=l_c/|\mathcal{T}|$	average term length