# Math 311 Auxiliary Homework

1. To accompany the HW due on Jan. 29

1. (a) Use the DeMorgan laws to argue that (A and B) ≡ (A or B).

(b) Use induction to show n! ≤ nn for every n ∈ N.

2. (a) Use the DeMorgan laws to argue that and

(b) Prove that the empty set is a subset of every set.

3. Inter pret in words : but not . (x, y are integers)

4. (a) Prove that using the geometric series formula :

Hint: use .

(b) Briefly explain why (1+b)2 > 1+2b in the proof of the Thm. in §1.4.1.

(c) Briefly explain why the conclusion of the proof of Thm. in §1.5.2
fol lows from the Archimedean Property of R .

2. To accompany the HW due on Feb.7

1. Prove n!≤nn for every n ∈ N, without using induction.

2. (a) Prove that for x, y ∈ R.

(b) Prove for .

3. Show that if r is a non zero rational number and x ∈ R \ Q (i.e., x is
irrational), then r + x and rx are both irrational.

3. To accompany the HW due on Feb.14

The first two problems use these two definitions.

Definition 1. A dyadic rational is a rational number of the form

, n ∈ Z, k = 0, 1, 2, ...

So, for example, are all dyadic rationals.

Definition 2. For A B R, we say A is dense in B iff between any two points
of B we can find a point of A, i.e.,

with x < a < y.

For example, we've seen that Q is dense in R. It is important that all inequalities
are strict in this definition.

1. Show that the dyadic rationals are dense in Q.

2. Show that density is transitive, i.e., that if A is dense in B and B is dense
in C, then A is dense in C. (Hint: in this situation, A B C R.)
Deduce that the dyadic rationals are dense in R.

3. Show that log n is increasing , using nothing but the fact that log x > 0 for
x > 1.

4. Show that .

4. To accompany the HW due on Feb. 21

1. iff every neighbourhood of x of the form ,
contains all but finitely many points .

2. Can you have a sequence which, for any given rational number p ∈ Q
has a subsequence ? Construct one or prove it is impossible.

3. Prove the equivalence of the two definitions of Cauchy sequence.

4. If is Cauchy in R and some subsequence converges to x ∈ R,
then prove the full sequence also converges to x.

5. To accompany the HW due on Feb. 28

No extra problems this week.

6. To accompany the HW due on Mar. 7

(1) If converges and is monotonic and bounded, then con-
verges.

(2) Give an example to show that you can have diverge and diverge,
but converges.

(3) (a) Show that if converges absolutely, then does, too. Is this
true without the hypothesis of absolute convergence?

(b) If converges and   ≥ 0, what can be said about ?

(4) (a) (Dirichlet's Test) Suppose that the partial sums form a
bounded sequence, and suppose there is a sequence with
and . Then show converges. (Hint:

(b) Use Dirichlet's test to prove the alternating series test.

(5) For an alternating series, , where .

7. To accompany the HW due on Mar. 14

(1) Define . Prove .

(2) For how many points x ∈R can a power series converge conditionally?
Prove it.

8. To accompany the HW due on Mar. 28

(1) An even function times an even function is even. An odd function times an
even function is odd. What about an odd function times an odd function?
( Compare with the rules for integers .)

(2) If f and g are bounded on I, show that f + g and fg are bounded on I.

(3) Show that if converges absolutely, then so does . Is this true
without the hypothesis of absolute convergence?

(4) Does converge?

9. To accompany the HW due on Apr. 4

(1) .

(2) (Pasting Lemma) Let f be continuous on [a, b] and g be continuous on [b, c].
If f(b) = g(b), then

is continuous on [a, c].

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