## Prime Number

A prime number is a Positive Integer which has no Divisors other than 1 and itself. Although the number 1 used to be considered a prime, it requires special treatment in so many definitions and applications involving primes greater than or equal to 2 that it is usually placed into a class of its own. Since 2 is the only Even prime, it is also somewhat special, so the set of all primes excluding 2 is called the Odd Primes.'' The first few primes are 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, ... (Sloane's A000040, Hardy and Wright 1979, p. 3). Positive Integers other than 1 which are not prime are called Composite.

The function which gives the number of primes less than a number is denoted and is called the Prime Counting Function. The theorem giving an asymptotic form for is called the Prime Number Theorem.

Prime numbers can be generated by sieving processes (such as the Eratosthenes Sieve), and Lucky Numbers, which are also generated by sieving, appear to share some interesting asymptotic properties with the primes.

Many Prime Factorization Algorithms have been devised for determining the prime factors of a given Integer. They vary quite a bit in sophistication and complexity. It is very difficult to build a general-purpose algorithm for this computationally hard'' problem, so any additional information which is known about the number in question or its factors can often be used to save a large amount of time. The simplest method of finding factors is so-called Direct Search Factorization'' (a.k.a. Trial Division). In this method, all possible factors are systematically tested using trial division to see if they actually Divide the given number. It is practical only for very small numbers.

Because of their importance in encryption algorithms such as RSA Encryption, prime numbers can be important commercial commodities. In fact, Roger Schlafly has obtained U.S. Patent 5,373,560 (12/13/94) on the following two primes (expressed in hexadecimal notation):

and

The Fundamental Theorem of Arithmetic states that any Positive Integer can be represented in exactly one way as a Product of primes. Euclid's Second Theorem demonstrated that there are an infinite number of primes. However, it is not known if there are an infinite number of primes of the form , whether there are an Infinite number of Twin Primes, or if a prime can always be found between and .

Prime numbers satisfy many strange and wonderful properties. For example, there exists a Constant known as Mills' Constant such that

 (1)

where is the Floor Function, is prime for all . However, it is not known if is Irrational. There also exists a Constant such that
 (2)

(Ribenboim 1996, p. 186) is prime for every .

Explicit Formulas exist for the th prime both as a function of and in terms of the primes 2, ..., (Hardy and Wright 1979, pp. 5-6; Guy 1994, pp. 36-41). Let

 (3)

for integral , and define , where is again the Floor Function. Then
 (4) (5)

where is the Prime Counting Function. It is also true that

 (6)

(Ribenboim 1996, pp. 180-182). Note that the number of terms in the summation to obtain the th prime is , so these formulas turn out not to be practical in the study of primes. An interesting Infinite Product formula due to Euler which relates and the th Prime is
 (7) (8)

(Blatner 1997). Conway (Guy 1983, Conway and Guy 1996, p. 147) gives an algorithm for generating primes based on 14 fractions, but it is actually just a concealed version of a Sieve.

Some curious identities satisfied by primes are

 (9)

 (10)

(Doster 1993),
 (11)

(Le Lionnais 1983, p. 46),
 (12)

and

 (13)

(Berndt 1994, p. 114).

It has been proven that the set of prime numbers is a Diophantine Set (Ribenboim 1991, pp. 106-107). Ramanujan also showed that

 (14)

where is the Prime Counting Function and is the Möbius Function (Berndt 1994, p. 117). B. M. Bredihin proved that
 (15)

takes prime values for infinitely many integral pairs (Honsberger 1976, p. 30). In addition, the function
 (16)

where
 (17)

is the Factorial, and is the Floor Function, generates only prime numbers for Positive integral arguments. It not only generates every prime number, but generates Odd primes exactly once each, with all other values being 2 (Honsberger 1976, p. 33). For example,
 (18) (19) (20)

with no new primes generated for .

For an Integer , is prime Iff

 (21)

for , 1, ..., (Deutsch 1996).

Cheng (1979) showed that for sufficiently large, there always exist at least two prime factors between and for (Le Lionnais 1983, p. 26). Let be the number of decompositions of into two or more consecutive primes. Then

 (22)

(Moser 1963, Le Lionnais 1983, p. 30). Euler showed that the sum of the inverses of primes is infinite
 (23)

(Hardy and Wright 1979, p. 17), although it diverges very slowly. The sum exceeds 1, 2, 3, ... after 3, 59, 361139, ... (Sloane's A046024) primes, and its asymptotic equation is
 (24)

where is Mertens Constant (Hardy and Wright 1979, p. 351). Dirichlet showed the even stronger result that
 (25)

(Davenport 1980, p. 34).

Despite the fact that diverges, Brun showed that

 (26)

where is Brun's Constant. The function defined by
 (27)

taken over the primes converges for and is a generalization of the Riemann Zeta Function known as the Prime Zeta Function.

The probability that the largest prime factor of a Random Number is less than is ln 2 (Beeler et al. 1972, Item 29). The probability that two Integers picked at random are Relatively Prime is , where is the Riemann Zeta Function (Cesaro and Sylvester 1883). Given three Integers chosen at random, the probability that no common factor will divide them all is

 (28)

where is Apéry's Constant. In general, the probability that random numbers lack a th Power common divisor is (Beeler et al. 1972, Item 53).

Large primes include the large Mersenne Primes, Ferrier's Prime, and (Cipra 1989). The largest known prime as of 1998, is the Mersenne Prime .

Primes consisting of consecutive Digits (counting 0 as coming after 9) include 2, 3, 5, 7, 23, 67, 89, 4567, 78901, ... (Sloane's A006510).

See also Adleman-Pomerance-Rumely Primality Test, Almost Prime, Andrica's Conjecture, Bertrand's Postulate, Brocard's Conjecture, Brun's Constant, Carmichael's Conjecture, Carmichael Function, Carmichael Number, Chebyshev Function, Chebyshev-Sylvester Constant, Chen's Theorem, Chinese Hypothesis, Composite Number, Composite Runs, Copeland-Erdös Constant, Cramer Conjecture, Cunningham Chain, Cyclotomic Polynomial, de Polignac's Conjecture, Dirichlet's Theorem, Divisor, Erdös-Kac Theorem, Euclid's Theorems, Feit-Thompson Conjecture, Fermat Number, Fermat Quotient, Ferrier's Prime, Fortunate Prime, Fundamental Theorem of Arithmetic, Gigantic Prime, Giuga's Conjecture, Goldbach Conjecture, Good Prime, Grimm's Conjecture, Hardy-Ramanujan Theorem, Irregular Prime, Kummer's Conjecture, Lehmer's Problem, Linnik's Theorem, Long Prime, Mersenne Number, Mertens Function, Miller's Primality Test, Mirimanoff's Congruence, Möbius Function, Palindromic Number, Pépin's Test, Pillai's Conjecture, Poulet Number, Primary, Prime Array, Prime Circle, Prime Factorization Algorithms, Prime Number of Measurement, Prime Number Theorem, Prime Power Symbol, Prime String, Prime Triangle, Prime Zeta Function, Primitive Prime Factor, Primorial, Probable Prime, Pseudoprime, Regular Prime, Riemann Function, Rotkiewicz Theorem, Schnirelmann's Theorem, Selfridge's Conjecture, Semiprime, Shah-Wilson Constant, Sierpinski's Composite Number Theorem, Sierpinski's Prime Sequence Theorem, Smooth Number, Soldner's Constant, Sophie Germain Prime, Titanic Prime, Totient Function, Totient Valence Function, Twin Primes, Twin Primes Constant, Vinogradov's Theorem, von Mangoldt Function, Waring's Conjecture, Wieferich Prime, Wilson Prime, Wilson Quotient, Wilson's Theorem, Witness, Wolstenholme's Theorem, Zsigmondy Theorem

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