This Mma program implements the definition of the above sequence but generates quite different terms: RecurrenceTable[{a[1]==a[2]==0,a[3]==1,a[n]==Abs[a[n-1]-a[n-2]]+(-1)^(n a[n-3])},a,{n,80}] Is the Mma program wrong, or are the terms of the sequence wrong? Best, Harvey P.S. In any event, given the definition of the sequence, shouldn't the offset be changed?