Euclid’s Lemma

An irrational number is any real number that cannot be expressed as a ratio of two integers, [latex]p/q[/latex], where [latex]p[/latex] is an integer and [latex]q[/latex] is a non-zero integer. In other words, they are real numbers that are not rational. Their decimal representation never terminates and never enters a permanently repeating pattern.

A real number is rational if and only if its decimal representation is periodic. This means the sequence of digits eventually repeats a finite sequence of digits indefinitely. This repeating part is called the repetend. For example, [latex]1/3 = 0.333...[/latex] (repetend is '3') and [latex]3/7 = 0.428571428571...[/latex] (repetend is '428571'). Terminating decimals are a special case where the repetend is '0'.

Bézout's theorem is a fundamental statement in intersection theory. It asserts that the number of intersection points of two plane algebraic curves of degrees [latex]m[/latex] and [latex]n[/latex] is exactly [latex]mn[/latex], provided that one works in a projective plane over an algebraically closed field, counts points with multiplicity, and includes points at infinity where parallel asymptotes meet.

The fundamental theorem of algebra states that every non-constant single-variable polynomial with complex coefficients has at least one complex root. This guarantees that the field of complex numbers is algebraically closed, meaning polynomial equations that cannot be solved in real numbers can be solved in complex numbers. For a polynomial [latex]p(z) = a_n z^n + \dots + a_1 z + a_0[/latex], there exists a [latex]z_0 in \mathbb{C}[/latex] such that [latex]p(z_0) = 0[/latex].

This theorem states that every integer greater than 1 is either a prime number or can be uniquely represented as a product of prime numbers, disregarding the order of the factors. For example, [latex]1200 = 2^4 \times 3^1 \times 5^2[/latex]. This unique factorization is a cornerstone of number theory, providing a fundamental multiplicative structure for the integers.

The canonical representation, or standard form, of a positive integer [latex]n[/latex] is its unique prime factorization written as a product of prime powers with the primes in increasing order. For any integer [latex]n > 1[/latex], it can be written as [latex]n = p_1^{a_1} p_2^{a_2} \cdots p_k^{a_k}[/latex], where [latex]p_1 < p_2 < \cdots < p_k[/latex] are prime numbers and the exponents [latex]a_i[/latex] are positive integers.

与柯西初值问题相关的偏微分方程的基本存在唯一性定理。该定理指出，如果偏微分方程和初始条件是“解析的”（可以用收敛幂级数表示），则在初始曲面的邻域中存在唯一解析解。它提供了局部存在性的保证，但并未解决全局行为或适定性问题。

Euclid's theorem states there are infinitely many prime numbers. The classic proof is by contradiction. It assumes a finite list of all primes [latex]p_1, p_2, \dots, p_n[/latex]. It then considers the number [latex]P = p_1 p_2 \cdots p_n + 1[/latex]. This number [latex]P[/latex] is either prime or not. If it's prime, it's a new prime not on the list.

A rational number is any number that can be expressed as a fraction or quotient [latex]p/q[/latex], where [latex]p[/latex] is an integer and [latex]q[/latex] is a non-zero integer. The set of all rational numbers is denoted by [latex]\mathbb{Q}[/latex]. This fundamental concept extends the integers to include fractions, allowing for the representation of parts of a whole.

A partial differential equation (PDE) is an equation that imposes relations between the various partial derivatives of a multivariable function. The function is often called the unknown, and a PDE describes a relationship between this unknown function and its derivatives. Unlike ordinary differential equations (ODEs), which involve functions of a single variable, PDEs are fundamental to modeling multidimensional systems.

一种求解一阶和双曲二阶偏微分方程 (PDE) 的技术。该方法将偏微分方程 (PDE) 简化为一系列沿着特定曲线（称为“特征”）的常微分方程 (ODE)。沿着这些曲线，PDE 会简化，从而可以通过对 ODE 组进行积分来求得解。该方法对于涉及传输和波传播的问题尤其有效。

A direct corollary of the fundamental theorem of algebra is that any non-constant polynomial with real coefficients can be factored into a product of linear factors and irreducible quadratic factors, all with real coefficients. The linear factors correspond to the real roots, while the irreducible quadratic factors correspond to pairs of complex conjugate roots [latex]a \pm bi[/latex].

A transcendental number is a real or complex number that is not algebraic, meaning it is not a root of any non-zero polynomial equation with integer (or rational) coefficients. All transcendental numbers are irrational, but not all irrational numbers are transcendental (e.g., [latex]\sqrt{2}[/latex] is irrational but algebraic, as it is a root of [latex]x^2 - 2 = 0[/latex]).

Despite being dense, meaning between any two distinct rational numbers there is another, the set of all rational numbers [latex]\mathbb{Q}[/latex] is countably infinite. This means that all rational numbers can be put into a one-to-one correspondence with the natural numbers [latex]\mathbb{N} = \{1, 2, 3, ...\}[/latex]. This surprising result demonstrates that [latex]\mathbb{Q}[/latex] has the same cardinality as [latex]\mathbb{N}[/latex] and [latex]\mathbb{Z}[/latex].