For a sequence u1,u2…, define Δ1(un)=un+1−un and, for all integer k>1, Δk(un)=Δ1(Δk−1(un)). If un=n3+n, then Δk(un)=0 for all n<spanclass=′latex−bold′>(A)</span>if k=1<spanclass=′latex−bold′>(B)</span>if k=2, but not if k=1<spanclass=′latex−bold′>(C)</span>if k=3, but not if k=2<spanclass=′latex−bold′>(D)</span>if k=4, but not if k=3<spanclass=′latex−bold′>(E)</span>for no value of k