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Number Theory Basics

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Fundamental Theorem of Arithmetic

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Every integer greater than 1 can be uniquely factorized into prime numbers. This theorem ensures that we can break down any number into a product of its prime factors in only one way, excluding the rearrangement of the prime factors.

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Pythagorean Triple

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A Pythagorean triple consists of three positive integers aa, bb, and cc, such that a2+b2=c2a^2 + b^2 = c^2. These are the integer solutions to the Pythagorean theorem for right triangles.

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Legendre's Conjecture

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Legendre's Conjecture suggests that there is at least one prime number between any two consecutive squares n2n^2 and (n+1)2(n+1)^2. This conjecture has not been proven or disproven.

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Lagrange's Four-Square Theorem

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The theorem states that every natural number can be represented as the sum of four integer squares. That is, for every n0n \geq 0, there exist integers x,y,z,tx, y, z, t such that n=x2+y2+z2+t2n = x^2 + y^2 + z^2 + t^2.

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Euler's Criterion

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Euler's Criterion is a statement in number theory that gives a condition for determining whether an integer is a quadratic residue modulo a prime. For an integer aa and an odd prime pp, aa is a quadratic residue modulo pp if and only if ap121(modp)a^{\frac{p-1}{2}} \equiv 1 \pmod p.

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Euler's Theorem

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Euler's Theorem is a generalization of Fermat's Little Theorem. It states that if nn is a positive integer and aa is an integer coprime to nn, then aφ(n)1(modn)a^{\varphi(n)} \equiv 1 \pmod n where φ\varphi is Euler's totient function.

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Dirichlet's Theorem on Arithmetic Progressions

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The theorem states that for any given two positive coprime integers aa and dd, there are infinitely many primes of the form a+nda + nd, where nn is a non-negative integer. This result shows that there are numerous numbers with the same difference between them that are prime.

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Carmichael's Theorem

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Carmichael's Theorem is a result in number theory that generalizes Fermat's little theorem. It states that for Carmichael numbers nn, for all integers aa that are coprime to nn, it holds that an11(modn)a^{n-1} \equiv 1 \pmod n.

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The Density of Primes

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The Density of Primes, given by the Prime Number Theorem, approximates the distribution of the primes by stating that the probability of a number around nn being prime is about 1ln(n)\frac{1}{\ln(n)}, where ln\ln is the natural logarithm.

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Euclid's Lemma

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If a prime pp divides the product abab of two integers aa and bb, then pp must divide at least one of those integers aa or bb. This is fundamental in the study of number theory because it leads to the unique factorization of integers.

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Fermat's Little Theorem

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If pp is a prime number, then for any integer aa, the number apaa^p - a is an integer multiple of pp. In the notation of modular arithmetic, this is expressed as apa(modp)a^p \equiv a \pmod p.

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Bezout's Identity

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Bezout's Identity states that for any non-zero integers aa and bb, there exist integers xx and yy such that ax+by=gcd(a,b)ax + by = \gcd(a, b), where gcd(a,b)\gcd(a, b) denotes the greatest common divisor of aa and bb.

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The Goldbach Conjecture

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The Goldbach Conjecture is one of the oldest and best-known unsolved problems in number theory and all of mathematics. It states that every even integer greater than 2 is the sum of two prime numbers.

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Gauss's Lemma

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In number theory, Gauss's Lemma relates to the Legendre symbol and gives a way to compute it. For a given odd prime pp and an integer aa not divisible by pp, the Legendre symbol (ap)(\frac{a}{p}) is equal to (1)n(-1)^n where nn is the number of integers kk such that p2<akmodp<p\frac{p}{2} < a k \mod p < p.

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Möbius Function

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The Möbius function μ(n)\mu(n) is a multiplicative function that is used in number theory and combinatorics. It is defined for a positive integer nn such that:

μ(n)={1if n is a square-free positive integer with an even number of prime factors1if n is a square-free positive integer with an odd number of prime factors0if n has a squared prime factor\mu(n) = \begin{cases} 1 & \text{if } n \text{ is a square-free positive integer with an even number of prime factors} \\ -1 & \text{if } n \text{ is a square-free positive integer with an odd number of prime factors} \\ 0 & \text{if } n \text{ has a squared prime factor} \end{cases}

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The Pell Equation

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The Pell Equation is a diophantine equation of the form x2ny2=1x^2 - ny^2 = 1 where nn is a non-square natural number. The solutions (x,y)(x, y) are called Pell numbers. This equation is central to the theory of quadratic forms and has an infinite number of solutions.

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The Partition Function

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In number theory, the Partition Function P(n)P(n) represents the number of distinct ways of representing nn as the sum of positive integers, without considering the order of addends. For example, P(4)=5P(4) = 5 because 4 can be partitioned as 4, 3+1, 2+2, 2+1+1, and 1+1+1+1.

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Fermat's Last Theorem

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Fermat's Last Theorem states that there are no three positive integers a,b,a, b, and cc such that an+bn=cna^n + b^n = c^n for any integer value of nn greater than 2. This was famously conjectured by Pierre de Fermat in 1637 and was only proven in 1994 by Andrew Wiles.

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Wilson's Theorem

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An integer p>1p > 1 is a prime number if and only if (p1)!+1(p - 1)! + 1 is divisible by pp where !! denotes the factorial operation. In modular arithmetic, this can be written as (p1)!1(modp)(p - 1)! \equiv -1 \pmod p.

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Euler's Totient Function

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Euler's Totient Function φ(n)\varphi(n) counts the number of positive integers less than or equal to nn that are coprime to nn. It is a key function in number theory, particularly in modular arithmetic and theorems involving congruences.

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The Division Algorithm

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Given two integers aa and bb, with b>0b > 0, there exists unique integers qq and rr such that a=bq+ra = bq + r and 0r<b0 \leq r < b. The number qq is called the quotient, while rr is the remainder.

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The Quadratic Reciprocity Law

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This is a theorem in number theory that allows the determination of the solvability of quadratic equations modulo prime numbers. It provides a relationship between the Legendre symbols (pq)(\frac{p}{q}) and (qp)(\frac{q}{p}) for any two odd primes pp and qq.

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Sophie Germain Prime

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A Sophie Germain prime is a prime number pp such that 2p+12p+1 is also prime. The number 2p+12p+1 is called a safe prime. These primes are named after French mathematician Sophie Germain, who used them in her work on Fermat's Last Theorem for the case n=5n=5.

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The Mersenne Prime

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A Mersenne prime is a prime number of the form Mn=2n1M_n = 2^n - 1 for some integer nn. Not all numbers of this form are prime, but primes of this form are used in large number cryptography.

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The Twin Prime Conjecture

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The Twin Prime Conjecture asserts that there are infinitely many pairs of prime numbers (p, p+2) such that both numbers are prime. A pair of primes is called a 'twin prime'.

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The Riemann Hypothesis

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The Riemann Hypothesis is a conjecture that the nontrivial zeroes of the Riemann zeta function all have real part 1/2. This hypothesis has profound implications on the distribution of prime numbers.

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Pigeonhole Principle

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The Pigeonhole Principle states that if nn items are put into mm containers, with n>mn > m, then at least one container must contain more than one item. This simple yet powerful principle is used in various mathematical proofs.

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Chinese Remainder Theorem

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If one knows the remainders of the division of an integer nn by several pairwise coprime integers, then one can determine uniquely the remainder of the division of nn by the product of these integers, up to the product of these integers.

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Ramsey's Theorem

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Ramsey's Theorem states that for any given positive integers rr and ss, there is a minimum number NN such that if the edges of a complete graph of at least NN vertices are colored with two colors, there is a monochromatic rr-subset or a monochromatic ss-subset. This theorem is a fundamental result in combinatorial number theory and graph theory.

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