cell s ". ▁ ▁It ▁can ▁be ▁finite ▁or ▁infinite ▁in ▁extent . ▁Each ▁cell ▁has ▁a ▁set ▁of ▁" ne igh b ors ". ▁In ▁the ▁simplest ▁case , ▁each ▁cell ▁has ▁four ▁neighb ors , ▁those ▁being ▁the ▁cells ▁directly ▁above ▁or ▁below ▁or ▁to ▁the ▁left ▁or ▁right ▁of ▁the ▁given ▁cell . ▁ ▁Like ▁this : ▁the ▁b ' s ▁are ▁all ▁of ▁the ▁neighb ors ▁of ▁the ▁a . ▁The ▁a ▁is ▁one ▁of ▁the ▁neighb ors ▁of ▁each ▁of ▁the ▁b ' s . ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ ▁▁▁▁ ▁b ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ ▁ ▁b ▁a ▁b ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ ▁▁▁ ▁b ▁ ▁At ▁each ▁" time " ▁t = 0 , 1 , 2 , 3 , .... , ▁each ▁cell ▁is ▁assigned ▁one ▁of ▁three ▁" states ", ▁typically ▁called ▁" rest ing " ▁ ▁( or ▁" qu ies cent " ▁; ▁see ▁exc itable ▁medium ), ▁" ex c ited ", ▁or ▁" ref ract ory ". ▁The ▁assignment ▁of ▁states ▁for ▁all ▁cells ▁is ▁arbitrary ▁for ▁t ▁= ▁ 0 , ▁and ▁then ▁at ▁subsequent ▁times ▁the ▁state ▁of ▁each ▁cell ▁is ▁determined ▁by ▁the ▁following ▁rules . ▁▁ 1 . ▁If ▁a ▁cell ▁is ▁in ▁the ▁excited ▁state ▁at ▁time ▁t ▁then ▁it ▁is ▁in ▁the ▁ref ract ory ▁state ▁at ▁time ▁t + 1 . ▁▁ 2 . ▁If ▁a ▁cell ▁is ▁in ▁the ▁ref ract ory ▁state ▁at ▁time ▁t ▁then ▁it ▁is ▁in ▁the ▁rest ing ▁state ▁at ▁time