1SAS Sequences and Series Autumn 2022 Problem Sheet 1
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1SAS
Sequences and Series
Problem Sheet 1
Q1 (i) Recall the following definition from the lectures:
Definition. A sequence of real numbers (an ) tends to infinity if given any real number A > 0 there exists N e N such that
an > A for all n > N.
Using the definition prove that the sequence (an ) given by
an =
tends to infinity.
n4 + 16n2 + 4 |
7n2 + 2n |
(ii) Recall the following definition from the lectures:
Definition. A sequence (an ) of real numbers converges to a real number é if given any ∈ > 0 there exists N e N such that
Ian - éI < ∈ for all n > N.
A sequence (an ) converges if it converges to é for some real number é . Using the definition prove that the sequence (an ) given by
an =
converges.
n |
2n + sin2 (n) |
Q2. Prove (using the definition) that the sequence (an ) tends to infinity in each of the
following cases:
(i) an = n1/4; (ii) an = .
Q3. For each of the following sequences (an ) and values of é, prove (using the definition) that an → é:
(i) an = , é = ;
(ii) an = , é = .
Q4. (i) Prove that if x, y > 0 then
^x - ^y = x - y
(ii) Using the definition of convergence, prove that the sequence (an ) given by an = ^n + 1 - ^n
converges to 0.
Q5. Suppose that é > 0 and that an → é . Prove that there exists N e N such that
an > 0 for all n > N .
ExTRA QUEsT1oNs
EQ1. Prove that if an → o and bn → o then (a) an + bn → o, and (b) an bn → o.
EQ2. Consider the sequence
1 - 1 1 - 1 1
which has nth term an = .
(i) Find a natural number N for which an e (- , ) for all n > N .
[Here (- , ) denotes the interval {x e R : - < x < }.] (ii) Find a natural number N for which an e (- , ) for all n > N .
(iii) Let ∈ be any positive real number. Find a natural number N for which an e (-∈, ∈) for all n > N . What does this prove?
EQ3. For each of the following sequences (an ) determine whether it converges or tends
to infinity. Use the definitions to prove any claims that you make.
(i) an = (2 + (-1)n )n,
(ii) an = ,
(iii) an = ,n2 + n - n.
EQ4. Suppose that N0 e N and that an 2 bn for all n > N0 . Prove that if bn → o then
an → o.
EQ5. Give explicit examples of sequences (an ) and (bn ), satisfying an → o and bn → 0,
for which
(i) an bn → 1.
(ii) an bn → 0.
(iii) an bn → o.
(iv) an bn → -o.
(v) the sequence (an bn ) neither converges nor tends to ±o.
(vi) bn > 0 for all n e N, and the sequence (an bn ) neither converges nor tends to
±o.
[Here (an bn ) is the sequence whose nth term is the product an bn .]
EQ6. Give explicit examples of sequences (an ) and (bn ), satisfying an → o and bn → o,
for which
(i) an - bn → 0,
(ii) an - bn → 1,
(iii) an - bn → o,
(iv) an - bn → -o,
(v) the sequence (an - bn ) neither converges nor tends to ±o.
[Here (an - bn ) is the sequence whose nth term is the difference an - bn .]
EQ7. Suppose that (an ) is a sequence of positive real numbers converging to é > 0.
(i) Prove that
I^an - ^éI <
for all n e N.
(ii) Using Part (i), or otherwise, prove that ^an → ^é as n → o.
(iii) Deduce that
^n4 + 4n
n2 + 1 ,
justifying any assertions that you make.
EQ8. (i) For which values of n e N is it true that 2n 2 n? Prove your assertion.
Using your result, or otherwise, prove that
2n → o.
(ii) For which values of n e N is it true that 2n 2 n2 ? Prove your assertion. Using your result, or otherwise, prove that
→ o.
(iii) Outline a potential strategy for proving that
→ o
for all k = 0, 1, 2, 3, . . ..
2022-10-14