Homework Help: Prove Sequence is Bounded

1. Oct 10, 2007

christianrhiley

Definitions: Let {x[n]} be a bounded sequence in Reals.
We define {y[k]} and {z[k]} by
y[k]=sup{x[n]: n $$\geq$$ k}, z[k]=inf{x[n]: n $$\geq$$ k}

Claim: (i) Both y[k] and z[k] are bounded sequences
(ii){y[k]} is a decreasing sequence
(iii){z[k]} is an increasing sequence

Proof: (i) suppose y[k] and z[k] are not bounded. this implies x[n] is unbounded, a contradiction. therefore, we conclude that both y[k] and z[k] are bounded.
(ii)Let S[k] = {x[n]: n $$\geq$$ k} and S[k+1] = {x[n]: n $$\geq$$ k + 1}
S[k] is a $$\subset$$ S[k+1], and if sup(S[k]) $$\leq$$ sup(S[k+1]), it follows that S[k] $$\leq$$ S[k+1]. We conclude that {y[k]} is decreasing.
(iii)similar to part (ii) except inf(S[k+1]) $$\leq$$ inf(S[k])

Note: inf(A) $$\leq$$ inf(B) and sup(B) $$\leq$$ sup(A) have already been proven in an earlier exercise.

Where I Need Help: I need input regarding all three parts. I have made, at best, an informal sketch of a proof, and I would like some input on how to turn it into a rigorous proof.

2. Oct 10, 2007

Dick

If you mean that B is a subset of A implies that sup(B)<=sup(A) and inf(B)>=inf(A), I really don't see the need for more 'rigor'. I think you have it.