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(Exercise (17), Section 3.2) Let \(a\) and \(b\) be natural numbers such that \(a^{2}=\) \(b^{3} .\) Prove each of the propositions in Parts (6a) through (6d). (The results of Exercise (1) and Theorem 3.10 from Section 3.2 may be helpful.) (a) If \(a\) is even, then 4 divides \(a\). (b) If 4 divides \(a,\) then 4 divides \(b\). (c) If 4 divides \(b,\) then 8 divides \(a\). (d) If \(a\) is even, then 8 divides \(a\). (e) Give an example of natural numbers \(a\) and \(b\) such that \(a\) is even and \(a^{2}=b^{3},\) but \(b\) is not divisible by 8.

Short Answer

Expert verified
In this exercise, we have proven the following propositions: (a) If \(a\) is even, then 4 divides \(a\). (b) If 4 divides \(a\), then 4 divides \(b\). (c) If 4 divides \(b\), then 8 divides \(a\). (d) If \(a\) is even, then 8 divides \(a\). Furthermore, we have provided an example where \(a\) is even, \(a^2 = b^3\), and \(b\) is not divisible by 8: \(a = 4\) and \(b = 2\).

Step by step solution

01

Part (a): If a is even, then 4 divides a.

Let \(a\) be an even number. That means we can represent \(a\) as \(a = 2k\), where \(k\) is an integer. We are given that \(a^2 = b^3\). Let's substitute the given value of \(a\) into this relation: \[(2k)^2 = b^3.\] Simplifying, we get: \[4k^2 = b^3.\] Since \(4k^2\) is divisible by 4, it implies that 4 divides \(a\).
02

Part (b): If 4 divides a, then 4 divides b.

Let \(a = 4n\), where \(n\) is an integer. We are given that \(a^2 = b^3\). Let's substitute the given value of \(a\) into this relation: \[(4n)^2 = b^3.\] Simplifying, we get: \[16n^2 = b^3.\] Since \(16n^2\) is divisible by 4, it implies that 4 divides \(b\).
03

Part (c): If 4 divides b, then 8 divides a.

Let \(b = 4m\), where \(m\) is an integer. We are given that \(a^2 = b^3\). Let's substitute the given value of \(b\) into this relation: \[a^2 = (4m)^3.\] Simplifying, we get: \[a^2 = 64m^3.\] Since \(a^2\) is divisible by 8 (since 64 is a multiple of 8), it implies that 8 divides \(a\).
04

Part (d): If a is even, then 8 divides a.

If \(a\) is even, then following the proofs of parts (a), (b), and (c), we have shown that 4 divides \(a\), 4 divides \(b\), and 8 divides \(a\). Therefore, if \(a\) is even, then 8 divides \(a\).
05

Part (e): Example where a is even, a^2 = b^3, and b is not divisible by 8.

Let's consider \(a = 4\) and \(b = 2\). These values satisfy the given relations: \(a\) is even and \(a^2 = b^3\), since \(4^2 = 16\) and \(2^3 = 8\). However, \(b\) is not divisible by 8, since \(b = 2\) and \(2 < 8\).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Divisibility Properties
Divisibility is a fundamental concept in number theory, dealing with the even distribution of integers. An integer a is divisible by another integer b if there exists an integer c such that a = b \times c. In simpler terms, a can be divided by b without leaving a remainder.

When considering exponential relationships like \(a^2 = b^3\), we can analyze the divisibility properties of a and b. If a is even, it can be expressed as \(2k\), and square of an even number is always divisible by 4 because \( (2k)^2 = 4k^2 \). Similarly, if 4 divides a, meaning a can be written as \(4n\), the cube of a will reflect this divisibility. To better understand, breaking down integers into prime factors can efficiently demonstrate their divisibility characteristics and how they hold up in equations or inequalities.
Proof by Contradiction
Proof by contradiction, also known as indirect proof, is a logical argument technique where the assumption of the statement being false leads to a contradiction. This type of proof follows a two-step process: first, we assume the opposite of what we want to prove is true, and second, we show that this assumption leads to an impossible situation or a contradiction.

To apply proof by contradiction to divisibility, we might assume that, contrary to what we want to prove, 4 does not divide a. Subsequently, we would demonstrate that this assumption contradicts the given conditions or known theorems, leading to the conclusion that our original statement must be true - that 4 does indeed divide a. Through logical deduction, this method is particularly powerful in number theory and can be elegantly applied to proofs concerning divisibility, revealing hidden relationships between numbers.
Even and Odd Numbers
Even and odd numbers are the two discrete categories that whole numbers can fall into based on their divisibility by 2. An even number is an integer that can be divided by 2 without any remainder (e.g., 4, 10, or 28), often represented as \(2k\), where k is an integer. An odd number, in contrast, has a remainder of 1 when divided by 2 (such as 3, 7, or 17), and can be represented as \(2m + 1\), where m is also an integer.

Understanding the distinction between even and odd numbers is crucial when solving problems in number theory. For example, the square of an even number results in an even number, hence understanding these properties simplifies the analysis of equations involving squares or cubes of numbers and their underlying divisibility characteristics.
Exponential Equality
Exponential equality takes place when two exponential expressions with potentially different bases and exponents are set equal to each other, such as \(a^2 = b^3\). It can offer insights into the properties of the involved variables. For instance, if we have an equation where a squared number equals a cubed number, we can infer relationships about the divisibility of the base numbers.

When dealing with an equation like this, it is often helpful to express the bases in terms of their prime factors. The properties of exponents can also be used to demonstrate the relationship between the variables. By analyzing the exponential equality, we can deduce further properties about the variables, such as demonstrating that if one base is divisible by a certain number, the other must be as well, which is elegantly showcased in the relationship and divisibility of even numbers in the given exercise.

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Most popular questions from this chapter

In Section \(3.1,\) we defined congruence modulo \(n\) where \(n\) is a natural number. If \(a\) and \(b\) are integers, we will use the notation \(a \neq b(\bmod n)\) to mean that \(a\) is not congruent to \(b\) modulo \(n\). * (a) Write the contrapositive of the following conditional statement: For all integers \(a\) and \(b,\) if \(a \neq 0(\bmod 6)\) and \(b \neq 0(\bmod 6),\) then \(a b \not \equiv 0(\bmod 6)\). (b) Is this statement true or false? Explain.

Are the following propositions true or false? Justify all your conclusions. If a biconditional statement is found to be false, you should clearly determine if one of the conditional statements within it is true. In that case, you should state an appropriate theorem for this conditional statement and prove it. (a) For all integers \(m\) and \(n, m\) and \(n\) are consecutive integers if and only if 4 divides \(\left(m^{2}+n^{2}-1\right)\) (b) For all integers \(m\) and \(n, 4\) divides \(\left(m^{2}-n^{2}\right)\) if and only if \(m\) and \(n\) are both even or \(m\) and \(n\) are both odd.

Consider the following proposition: For each integer \(a, a \equiv 2(\bmod 8)\) if and only if \(\left(a^{2}+4 a\right) \equiv 4(\bmod 8)\) (a) Write the proposition as the conjunction of two conditional statements. (b) Determine if the two conditional statements in Part (a) are true or false. If a conditional statement is true, write a proof, and if it is false, provide a counterexample. (c) Is the given proposition true or false? Explain.

(a) Is the base 2 logarithm of \(32, \log _{2}(32),\) a rational number or an irrational number? Justify your conclusion. (b) Is the base 2 logarithm of \(3, \log _{2}(3),\) a rational number or an irrational number? Justify your conclusion.

(a) Use the result in Proposition 3.33 to help prove that the integer \(m=\) 5,344,580,232,468,953,153 is not a perfect square. Recall that an integer \(n\) is a perfect square provided that there exists an integer \(k\) such that \(n=k^{2} .\) Hint: Use a proof by contradiction. (b) Is the integer \(n=782,456,231,189,002,288,438\) a perfect square? Justify your conclusion.

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