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Formula For PrimesIn mathematics, it is known that no non-constant polynomial function P(n) exists that evaluates to a prime number for all integers n (or even almost all n). Using algebraic number theory one can make that quite precise. The quadratic polynomial - P(n) = n2 + n + 41
is prime for all non-negative integers less than 40. The primes for n = 0, 1, 2, 3... are 41, 43, 47, 53, 61, 71... The differences between the terms are 2, 4, 6, 8, 10... For n = 40, it produces a square number, 1681, which is equal to 41×41, the smallest composite number for this formula. In fact if 41 divides n it divides P(n) too. The phenomenon is related to the Ulam spiral, which is also implicitly quadratic, and the class number. A set of diophantine equations in 26 variables can be used to obtain primes: A given number k + 2 is prime iff the following system of diophantine equations has a solution in the natural numbers (Riesel, 1994): -
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The following function yields all the primes, and only primes, for non-negative integers n: -
This formula is based on Wilson's theorem; the number two is generated many times and all other primes are generated exactly once by this function. (In fact a prime p is generated for n = (p − 1) and 2 otherwise (ie. when n + 1 is composite)) Using the floor function (defined to be the largest integer less than or equal to the real number x), one can construct several formulas for the n-th prime. These formulas are also based on Wilson's theorem and have little practical value: the methods mentioned above under "Finding prime numbers" are much more efficient. Define -
{ \sin^2 ( {\pi \over j} (j-1)!^2 ) } { \sin^2( {\pi \over j} ) } or, alternatively, -
These definitions are equivalent; π(m) is the number of primes less than or equal to m. The n-th prime number pn can then be written as -
Another approach does not use factorials and Wilson's theorem, but also heavily employs the floor function (S. M. Ruiz 2000): first define -
and then -
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