Table 2.

Notations used in the four-layer BiLSTM module

Notations	Implications
St	The state of the current BiLSTM cell
it	The new information getting into the BiLSTM cell state
σ	The Sigmoid function
ft	The forgotten information in the BiLSTM cell state
Xt	The input of the current BiLSTM cell
bi, bS, bf, b0	The bias in three gates of the current BiLSTM cell
Wi, WS, Wf	The weights in three gates of the current BiLSTM cell
Ot	The output information in the BiLSTM cell state
ht − 1, ht	The output of the previous and current BiLSTM cell
$h_t=\left[\overrightarrow{h_t},;,\overleftarrow{h_t}\right]$	Two vectors (forward and backward) to form the hidden state of a BiLSTM network