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# A. Hints and Answers

Exercise 2.11:
In each case there is only one invertible element.

Exercise 2.40:
Note that the calculator sum of a very small number and a very large number is the large number. Note that the calculator product of two small positive numbers is 0.

Exercise 2.55:
The system fails two axioms. The other three each fail one axiom.

Exercise 2.78:
Use (2.75).

Exercise 2.90:
For part (b), use (2.74).

Exercise 2.93:
Part e) can be done quickly by using parts a), b) and d).

Exercise 2.123:
One of the conditions is that and have the same sign.

Exercise 2.135:
There are nine cases to consider (three for and three for ). They can be reduced to five cases, one of which is ( or .

Exercise 2.144:
There is a very short proof for a).

Exercise 2.145:
Apply the product formula for absolute values to .

Exercise 2.154:
Use (2.138).

Exercise 3.24:
You can take .

Exercise 3.32:
Suppose is both even and odd, and derive a contradiction by using theorems 3.15 and 3.19.

Exercise 3.43:
For part b) use part a) together with exercise 3.32

Exercise 3.57:
You can let  '' or
 ''.

Exercise 3.82:
f) Note that .

Exercise 3.84:
.

Exercise 3.86:
.

Exercise 4.19:
a) Note that . b) The solutions are and .

Exercise 4.23:
For part c), write and use part b). Remark 4.22 is used for part f).

Exercise 4.25:
a) 1. b) 32i.

Exercise 5.15:
I let  and '' and I used the fact that . (It would be reasonable to assume for all , but I proved it).

Exercise 5.48:
Suppose , and use trichotomy to show that .

Exercise 5.51:
a) If , then . b) If , then . (We know the laws of exponents for integer exponents.)

Exercise 5.54:
Raise both sides of the equation to the same integer power, and use laws of exponents for integer powers.

Exercise 6.5:
e) Notice that , and use theorem 2.128.

Exercise 6.26:
The roots are .

Exercise 7.21:
I showed , so that . Then I used the fact that is known to be a null sequence, and used the root theorem and the comparison theorem.

Exercise 7.22:
All three sequences are null sequences. For the last sequence, I showed for all . For the first sequence , I showed for all .

Exercise 7.24:
The set of all complex sequences does not form a field. All of the field axioms except one hold.

Exercise 7.28:
works.

Exercise 7.44:
You can take , where for all .

Exercise 7.49:
Note that , and show that the assumption converges is contradictory.

Exercise 7.50:
Note that if , then .

Exercise 7.56:
Multiply numerator and denominator of by .

Exercise 7.57:
All of the sequences converge. and .

Exercise 7.58:
If is a bound for , and is a bound for , then is a bound for .

Exercise 7.60:
A convergent sequence is the sum of a null sequence and a constant sequence. Now use exercise 7.58.

Exercise 7.69:
.

Exercise 7.70:
One of the sequences diverges. One of the limits is .

Exercise 7.74:
Apply theorem 7.7 to .

Exercise 7.75:
The statement is false.

Exercise 7.86:
The sequence converges to or to or else it diverges. (You should find exact conditions on , , , and corresponding to each case.)

Exercise 7.94:
If for all , then either or will be a bound for

Exercise 7.100:
Use corollary 7.96 to show that the sequence converges. Note . Use the translation theorem to show that the limit is .

Exercise 8.4:
I used the polar decomposition for .

Exercise 8.19:
Yes. In fact every function from to is continuous. The only integer that satisfies is zero.

Exercise 8.43:
a) Limit does not exist. b) Let be a sequence converging to . Apply null-times-bounded theorem to . c) I looked at sequences , where is a direction.

Exercise 9.10:
Four of the requested five functions exist. The other one doesn't.

Exercise 9.13:
Apply theorem 9.11 to , where

Exercise 9.17:
All four functions exist. I described by drawing its graph.

Exercise 9.19:
Assume and derive a contradiction from the intermediate value theorem.

Exercise 10.10:
is nowhere differentiable. The same sequences that show complex conjugation is nowhere differentiable show that is also.

Exercise 10.17:
.

Exercise 10.19:
Write , and use the reciprocal rule and exercise 10.18.

Exercise 10.20:
Use the product rule and the reciprocal rule.

Exercise 10.31:
Only one of the three statements is true.

Exercise 10.37:
Use the result of exercise 6.36

Exercise 10.50:
For both parts, compare with

Exercise 10.51:
The exact value of is . You can't prove this (because we have not yet defined ), but you can check your answer using this.

Exercise 11.19:
c) Note that the comparison test holds only for sequences of non-negative numbers.

Exercise 11.28:
In most cases there is one value for which the ratio test gives no information. However the exceptional values are usually standard test series. For part g) the exceptional value was considered in example 11.27.

Exercise 11.48:
b) Show that the real and imaginary parts of the series both converge.

Exercise 11.49:
Both series converge absolutely for and diverge for . (I used the ratio test.)

Exercise 12.6:
a) If then for all , and if , then for all . Except for the first few terms, both series are geometric series,

Exercise 12.14:
a) Consider geometric series of the form . c) Cf exercise 12.5.a

Exercise 12.20:
The sums are and Get the second by differentiating the first.

Exercise 12.33:
a) Use induction and (12.29). b) Use part a) and (12.28).

Exercise 12.34:
Calculate using exercise 12.33

Exercise 12.42:
a) Write and use (12.28). b) Write and use exercise 12.34.

Exercise 12.51:
Write the trig functions in terms of exponentials, and use several times.

Exercise 12.59:
Everything follows from and and .

Exercise 12.70:
.

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