▁" ref ract ory ", ▁and ▁the ▁" rest " ▁states . ▁However , ▁in ▁the ▁next ▁paragraph ▁they ▁ref ine ▁this ▁by ▁ident ifying ▁the ▁rest ▁state ▁with ▁the ▁number ▁ 1 , ▁the ▁active ▁state ▁with ▁the ▁number ▁ 0 , ▁and ▁the ▁ref ract ory ▁state ▁with ▁the ▁open ▁interval ▁( 0 , 1 ) ▁on ▁the ▁real ▁line . ▁The ▁number ▁assigned ▁to ▁a ▁give ▁point ▁at ▁a ▁given ▁time ▁is ▁called ▁its ▁" ep och ▁number " ▁at ▁that ▁time . ▁Thus , ▁the ▁" state ▁space " ▁is ▁the ▁interval ▁[ 0 , 1 ]. ▁And ▁in ▁the ▁next ▁paragraph ▁we ▁find ▁" .. be hind ▁every ▁wave ▁front ▁moving ▁freely ▁there ▁will ▁be ▁a ▁band ▁of ▁fixed ▁width ▁within ▁which ▁the ▁recovery ▁process ▁is ▁taking ▁place ." ▁A ▁wave ▁therefore ▁consists ▁of ▁a ▁" front ", ▁a ▁smooth ▁curve ▁of ▁points ▁with ▁epo ch ▁number ▁ 0 ▁which ▁moves ▁in ▁the ▁plane ▁with ▁constant ▁speed , ▁followed ▁by ▁a ▁ref ract ory ▁region ▁of ▁points ▁with ▁epo ch ▁number ▁in ▁( 0 , 1 ), ▁of ▁constant ▁width ▁( dep ending ▁on ▁the ▁velocity ), ▁and ▁leaving ▁behind ▁a ▁rest ▁region ▁of ▁points ▁with ▁epo ch ▁number ▁ 1 . ▁This ▁is ▁far ▁from ▁a ▁cell ular ▁autom ata , ▁and ▁is ▁more ▁correctly ▁called ▁a ▁" ge ometric " ▁model . ▁ ▁Further ▁on ▁in ▁ ▁it ▁is ▁stated ▁that ▁ ▁" Autom aton ▁models ▁of ▁exc itable ▁media ▁were ▁investig ated ▁in ▁[ 9