Real Numbers: Integers, Rational, and Irrational

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Questions and Answers

Though often glossed over, the demonstration that the set of algebraic numbers is countably infinite, while the set of real numbers is uncountably infinite, rigorously establishes the existence of ______ real numbers, a cornerstone concept in advanced real analysis.

transcendental

In the context of non-Diophantine approximations, demonstrating that a given irrational number, $\alpha$, has an unbounded partial quotient sequence in its continued fraction expansion implies that there exists no constant $k > 0$ such that $| \alpha - \frac{p}{q} | > \frac{k}{q^2}$ for all rational numbers $\frac{p}{q}$, thereby characterizing its ______.

irrationality

Given the field extension $\mathbb{Q}(\sqrt[3]{2})$ over $\mathbb{Q}$, demonstrating that the minimal polynomial of $\sqrt[3]{2}$ over $\mathbb{Q}$ is $x^3 - 2$ through Eisenstein's criterion guarantees that $\sqrt[3]{2}$ is not an element of $\mathbb{Q}$, thereby rigidly establishing its ______.

irrationality

Within the framework of p-adic numbers, a number is classified as a p-adic integer if its p-adic norm is less than or equal to 1; consequently, by demonstrating that a real number's p-adic approximation via Hensel's Lemma converges to a solution within the p-adic integers, one is intrinsically working with an element that, while potentially irrational in \mathbb{R}, possesses a specific ______ structure in \mathbb{Q}_p.

<p>algebraic</p> Signup and view all the answers

Consider a Dedekind cut $\alpha = (A, B)$ in the rational numbers; demonstrating that neither $A$ contains a largest element nor $B$ contains a smallest element, while also ensuring that $A$ and $B$ are non-empty and their union equals $\mathbb{Q}$, ensures that $\alpha$ represents a(n) ______ real number.

<p>irrational</p> Signup and view all the answers

Establishing that a convergent sequence of rational numbers, ${q_n}_{n=1}^{\infty}$, converges to a limit $L$ that, when incorporated into Liouville's approximation theorem, violates the condition that for any positive integer $n$, there exist integers $p$ and $q$ with $q > 1$ such that $|L - \frac{p}{q}| < \frac{1}{q^n}$, would categorically imply that $L$ is a(n) ______ number.

<p>transcendental</p> Signup and view all the answers

Given the algebraic closure $\overline{\mathbb{Q}}$ of the rational numbers $\mathbb{Q}$ within the complex numbers $\mathbb{C}$, a complex number $\alpha \in \mathbb{C}$ that demonstrably lies outside of $\overline{\mathbb{Q}}$meaning it is not a root of any non-zero polynomial equation with coefficients in $\mathbb{Q}$is, by definition, a(n) ______ number.

<p>transcendental</p> Signup and view all the answers

In the setting of non-standard analysis, demonstrating that a hyperreal number $x$ is infinitely close to a real number $r$, denoted as $x \approx r$, while also verifying that $x$ is not equal to $r$ (i.e., $x - r \neq 0$), implies that $x - r$ is a(n) ______, an element that exemplifies the distinction between the hyperreals and the reals.

<p>infinitesimal</p> Signup and view all the answers

If one is operating within the surreal numbers and constructs a number $\omega$ (omega) such that $\omega$ is greater than any standard integer $n \in \mathbb{Z}$, and subsequently demonstrates that $\frac{1}{\omega}$ is a positive number smaller than any positive standard real number $r \in \mathbb{R}_{>0}$, then $\frac{1}{\omega}$ is characterized as a(n) ______, embodying the extensions beyond the standard real numbers.

<p>infinitesimal</p> Signup and view all the answers

Consider the construction of real numbers via Cauchy sequences of rational numbers; if a Cauchy sequence ${q_n}_{n=1}^{\infty}$ is demonstrated to converge but not to any rational numberthat is, its limit $L$ (in the completion of $\mathbb{Q}$) is provably not an element of $\mathbb{Q}$then $L$ is necessarily categorized as a(n) ______ number by this construction.

<p>irrational</p> Signup and view all the answers

When applying Gelfond's theorem, proving that a number of the form $\alpha^\beta$ is transcendental necessitates that $\alpha$ is an algebraic number not equal to 0 or 1, and that $\beta$ is an ______ algebraic number, underscoring the theorem's conditions for transcendental number generation.

<p>irrational</p> Signup and view all the answers

Given a polynomial $P(x)$ with integer coefficients, characterized by $P(x) = a_n x^n + a_{n-1} x^{n-1} + \cdots + a_1 x + a_0$, where $a_i \in \mathbb{Z}$ for all $i$, demonstrating through the rational root theorem that it possesses no rational roots intrinsically establishes that any root of $P(x)$should it exist within the real numbersmust be necessarily ______, provided $P(x)$ is irreducible over $\mathbb{Q}$.

<p>irrational</p> Signup and view all the answers

In the context of algebraic number theory, if an algebraic integer $\alpha$ is proven to have a minimal polynomial over $\mathbb{Q}$ with a degree strictly greater than 1, it necessarily follows that $\alpha$ cannot be a rational number. Therefore, $\alpha$'s existence unequivocally classifies it as a(n) ______ algebraic number.

<p>irrational</p> Signup and view all the answers

Consider an integral domain $D$. If an element $a \in D$ has no multiplicative inverse within $D$, demonstrating that there does not exist any $b \in D$ such that $ab = 1_D$, and if $a$ is not the zero element ($a \neq 0_D$), then the element $a$ being expressible in the form $p/q$ where $p, q \in \mathbb{Z}$ necessitates that, within the scope of real numbers, $a$ is a(n) ______ number.

<p>rational</p> Signup and view all the answers

Suppose one employs the Euclidean algorithm to compute the greatest common divisor (GCD) of two integers, $a$ and $b$, specifically when aiming to demonstrate that the equation $ax + by = c$ has integer solutions for $x$ and $y$; if one finds that $c$ is not a multiple of $\text{GCD}(a, b)$, it follows that solutions within the field of integers do not exist, and the numbers involved (specifically $x$ and $y$) might necessitate analysis within the domain of ______ numbers, if solutions are sought in a broader number system.

<p>rational</p> Signup and view all the answers

Given the task of approximating the value of $\pi$ using Monte Carlo methods, whereby random points are uniformly generated within a square circumscribing a circle, demonstrating that the ratio of points falling inside the circle to the total points converges (as the number of points approaches infinity) to $\frac{\pi}{4}$ allows one to estimate $\pi$. However, the true value of $\pi$ remains inherently ______, irrespective of the precision achieved through statistical approximations.

<p>irrational</p> Signup and view all the answers

Within the context of metric spaces, demonstrating that the completion of the rational numbers $\mathbb{Q}$ under the standard Euclidean metric yields the real numbers $\mathbb{R}$ (i.e., $\overline{\mathbb{Q}} = \mathbb{R}$) inherently introduces numbers that are limits of Cauchy sequences of rationals but are not themselves rational; these newly incorporated numbers are fundamentally ______, showcasing the incompleteness of $\mathbb{Q}$.

<p>irrational</p> Signup and view all the answers

When analyzing the ring of Gaussian integers $\mathbb{Z}[i] = {a + bi : a, b \in \mathbb{Z}}$, identifying a Gaussian primean element that cannot be written as the product of two non-unit Gaussian integersand then demonstrating that the norm of this Gaussian prime (i.e., $a^2 + b^2$ for $a + bi$) is a prime number in $\mathbb{Z}$ congruent to 1 modulo 4, it implies that the original prime in $\mathbb{Z}$ is expressible as a sum of two squares. However, the numbers $a$ and $b$ representing the Gaussian integer components remain strictly ______, as elements of $\mathbb{Z}$.

<p>integers</p> Signup and view all the answers

In the landscape of formal language theory, if one constructs a Turing machine designed to enumerate all possible strings that can be generated from a given alphabet, and if demonstrating that the set of these strings is countably infinite, while concurrently showing that the set of all possible real numbers between 0 and 1 is uncountably infinite suggests that, there invariably exists real numbers whose digit sequences cannot be generated or recognized by any Turing machine; such numbers are consequently indicative of a class beyond algorithmic constructibility, and thereby have ______ digits.

<p>irrational</p> Signup and view all the answers

Consider a scenario in computational complexity where one aims to determine whether a given computational problem is solvable in polynomial time (i.e., within the complexity class P) or is NP-complete. If formulating a problem that requires representing numerical inputs, and demonstrating that the problem becomes NP-complete as the magnitude of these numerical inputs grows ______, it highlights the importance of number representation in complexity analysis.

<p>exponentially</p> Signup and view all the answers

Imagine one is working with a smooth dynamical system and aims to classify the long-term behavior of its trajectories. If one identifies an invariant torus within the phase space and demonstrates that the frequencies associated with motion on this torus are ______ (i.e., there exists no linear combination with integer coefficients that equals zero), implying that trajectories densely fill the torus over timea principle known as ergodicity on the torus.

<p>incommensurate</p> Signup and view all the answers

Consider a hypothetical scenario in cryptography where creating a public-key cryptosystem relies on the difficulty of factoring large ______ (products of two large prime numbers); if demonstrating the existence of a quantum algorithm (such as Shor's algorithm) that can factor such numbers in polynomial time undermines the security of the cryptosystem, emphasizing the importance of underlying mathematical assumptions in cryptographic design.

<p>semiprimes</p> Signup and view all the answers

Within the realm of fractal geometry, examining the Mandelbrot setdefined as the set of complex numbers $c$ for which the sequence $z_{n+1} = z_n^2 + c$ does not diverge from $z_0 = 0$and demonstrating that its boundary contains infinitely intricate details at every scale inherently involves representing complex numbers, which are formed from real and imaginary components; these components, when not expressible as a ratio of integers, result in complex numbers that, due to the irrational component, extend beyond mere ______.

<p>integers</p> Signup and view all the answers

When considering the Riemann zeta function, $\zeta(s) = \sum_{n=1}^{\infty} \frac{1}{n^s}$, and attempting to prove the Riemann hypothesis (which posits that all non-trivial zeros of $\zeta(s)$ have a real part equal to $\frac{1}{2}$), it underscores the intricate nature of zeta-function zeros. Demonstrating that $\zeta(s)$ exhibits non-trivial zeros away from the critical line ($\Re(s) = \frac{1}{2}$) would violate the Riemann hypothesis, emphasizing the complexity of how primes distribute, which directly relates to whether these zeros are real or ______.

<p>irrational</p> Signup and view all the answers

Suppose one has a dataset consisting of real values obtained from physical measurements. If, upon conducting statistical analysis, demonstrating that the residuals (i.e., the differences between observed and predicted values) conform to a normal distribution with a mean of zero indicates that the model adequately captures the underlying phenomena. Such residuals intrinsically are ______ numbers (real numbers) with a mean of zero.

<p>real</p> Signup and view all the answers

Flashcards

Real Numbers

All numbers representable on a number line, including both rational and irrational numbers.

Integers

Whole numbers (positive, negative, or zero) without fractions or decimals.

Rational Numbers

Numbers expressible as a fraction p/q, where p and q are integers and q ≠ 0.

Irrational Numbers

Numbers that cannot be expressed as a fraction p/q; have non-repeating, non-terminating decimal representations.

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Terminating Decimals

Decimals with a finite number of digits after the decimal point.

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Repeating Decimals

Decimals with a repeating pattern of digits that continues indefinitely.

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Closure (Integers)

Integers are closed under these operations; the result is always another integer.

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Closure (Rational Numbers)

Rational numbers are closed under these operations (except division by zero).

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Closure (Irrational Numbers)

Irrational numbers are generally not closed under these operations.

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Commutativity

The property that the order of numbers does not affect the result of addition or multiplication.

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Associativity

The property that the grouping of numbers does not affect the result of addition or multiplication.

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Distributivity

The property that a * (b + c) = a * b + a * c.

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Converting Terminating Decimals

Write as fraction with denominator of 10, 100, 1000, etc., then simplify.

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Converting Repeating Decimals

Use algebraic manipulation to eliminate the repeating part, then solve for x.

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Rational Number (Identification)

If a number can be written as p/q it is...

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Rational Number (Decimal ID)

If a decimal terminates or repeats it is…

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Irrational Number (Identification)

If a decimal is non-repeating and non-terminating it is...

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Rational + Rational

Rational plus Rational equal to...

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Rational + Irrational

Rational plus Irrational equal to...

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Irrational + Irrational

Irrational plus Irrational could be...

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Rational * Rational

Rational times Rational equal to...

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Rational * Irrational

Rational times Irrational equal to...

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Irrational * Irrational

Irrational times Irrational equal to...

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Proof that √2 is Irrational

A proof strategy by contradiction to show √2 can't be rational.

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Representing Irrational Numbers

Use estimation to approximately locate the point on the number line.

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Study Notes

  • Real numbers encompass all numbers that can be represented on a number line
  • This includes both rational and irrational numbers

Integers

  • Integers are whole numbers, which can be positive, negative, or zero
  • They do not include fractions or decimals
  • Examples of integers: -3, -2, -1, 0, 1, 2, 3
  • The set of integers is denoted by the symbol 'Z'

Rational Numbers

  • Rational numbers can be expressed as a fraction p/q, where p and q are integers, and q is not zero
  • All integers are rational numbers because any integer n can be written as n/1
  • Decimals that either terminate or repeat are rational numbers
  • Terminating decimals have a finite number of digits after the decimal point (e.g., 0.25 = 1/4)
  • Repeating decimals have a pattern of digits that repeats indefinitely (e.g., 0.333... = 1/3)

Irrational Numbers

  • Irrational numbers cannot be expressed as a fraction p/q, where p and q are integers
  • These numbers have non-repeating, non-terminating decimal representations
  • Common examples of irrational numbers include:
  • √2 (square root of 2) ≈ 1.41421356...
  • Ï€ (pi) ≈ 3.14159265...
  • e (Euler's number) ≈ 2.71828182...
  • Irrational numbers, when written as decimals, continue infinitely without repeating any pattern

Relationship between Number Types

  • Real Numbers: The overarching set containing both rational and irrational numbers
  • Rational Numbers: A subset of real numbers that can be expressed as a fraction
  • Integers: A subset of rational numbers that are whole numbers (positive, negative, or zero)
  • Every integer is a rational number, and every rational number is a real number, but the reverse is not always true
  • Irrational numbers are real numbers but are not rational

Key Properties

  • Closure:
    • Integers are closed under addition, subtraction, and multiplication (the result of these operations on integers is always an integer)
    • Rational numbers are closed under addition, subtraction, multiplication, and division (excluding division by zero)
    • Irrational numbers are not closed under these operations (e.g., √2 + (-√2) = 0, which is rational)
  • Commutativity:
    • Both rational and irrational numbers follow commutative properties for addition and multiplication (a + b = b + a, a * b = b * a)
  • Associativity:
    • Both rational and irrational numbers follow associative properties for addition and multiplication ((a + b) + c = a + (b + c), (a * b) * c = a * (b * c))
  • Distributivity:
    • Rational and irrational numbers follow the distributive property (a * (b + c) = a * b + a * c)

Examples

  • Classifying Numbers:
    • 5 is an integer, a rational number (5/1), and a real number
    • -3 is an integer, a rational number (-3/1), and a real number
    • 0.75 is a rational number (3/4) and a real number
    • 0.333... is a repeating decimal, thus a rational number (1/3), and a real number
    • √5 is an irrational number (approximately 2.236...), and a real number
    • Ï€/2 is an irrational number, and a real number

Converting Decimals to Fractions

  • Terminating Decimals:
    • Write the decimal as a fraction with a denominator of 10, 100, 1000, etc., depending on the number of decimal places
    • Simplify the fraction if possible
    • Example: 0.25 = 25/100 = 1/4
  • Repeating Decimals:
    • Let x equal the repeating decimal
    • Multiply x by 10, 100, 1000, etc., such that one repeating block is to the left of the decimal point
    • Subtract the original equation from the new equation to eliminate the repeating part
    • Solve for x
    • Example: Convert 0.333... to a fraction
    • Let x = 0.333...
    • 10x = 3.333...
    • 10x - x = 3.333... - 0.333...
    • 9x = 3
    • x = 3/9 = 1/3

Identifying Rational and Irrational Numbers

  • Key Indicators:
    • If a number can be written as a fraction p/q (where p and q are integers, q ≠ 0), it is rational
    • If a decimal terminates or repeats, it is rational
    • If a number is a non-repeating, non-terminating decimal, it is irrational

Operations Involving Rational and Irrational Numbers

  • Adding or Subtracting:
    • Rational + Rational = Rational
    • Rational + Irrational = Irrational
    • Irrational + Irrational = Could be either rational or irrational (e.g. √2 + (-√2) = 0 (rational), √2 + √3 = irrational)
  • Multiplying or Dividing:
    • Rational * Rational = Rational
    • Rational * Irrational = Irrational (if the rational number isn't zero)
    • Irrational * Irrational = Could be either rational or irrational (e.g. √2 * √2 = 2 (rational), √2 * √3 = √6 (irrational))

Examples of Proofs Involving Irrational Numbers

  • Proof that √2 is Irrational:
    • Assume √2 is rational, so √2 = p/q, where p and q are integers with no common factors (fraction in simplest form)
    • Square both sides: 2 = p²/q²
    • Multiply by q²: 2q² = p²
    • This means p² is even, and therefore p is even (since the square of an odd number is odd)
    • Let p = 2k, where k is an integer
    • Substitute into the equation: 2q² = (2k)² = 4k²
    • Divide by 2: q² = 2k²
    • This means q² is even, and therefore q is even
    • Since both p and q are even, they have a common factor of 2, which contradicts our initial assumption that p/q is in simplest form
    • Therefore, √2 is irrational

Representing Numbers on the Number Line

  • Integers: Represented by equally spaced points on the number line
  • Rational Numbers: Can be precisely located on the number line by dividing the space between integers based on the fraction
  • Irrational Numbers: Their approximate locations can be marked on the number line using decimal approximations

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