Primorial


In mathematics, and more particularly in number theory, primorial, denoted by “#”, is a function from natural numbers to natural numbers similar to the factorial function, but rather than successively multiplying positive integers, the function only multiplies prime numbers.
The name "primorial", coined by Harvey Dubner, draws an analogy to primes similar to the way the name "factorial" relates to factors.

Definition for prime numbers

For the th prime number, the primorial is defined as the product of the first primes:
where is the th prime number. For instance, signifies the product of the first 5 primes:
The first five primorials are:
The sequence also includes as empty product. Asymptotically, primorials grow according to:
where is Little O notation.

Definition for natural numbers

In general for a positive integer, a primorial can also be defined, namely as the product of those primes ≤ :
where is the prime-counting function, giving the number of primes ≤. This is equivalent to:
For example, 12# represents the product of those primes ≤ 12:
Since, this can be calculated as:
Consider the first 12 values of :
We see that for composite every term simply duplicates the preceding term, as given in the definition. In the above example we have since 12 is a composite number.
Primorials are related to the first Chebyshev function, written or according to:
Since asymptotically approaches for large values of, primorials therefore grow according to:
The idea of multiplying all known primes occurs in some proofs of the infinitude of the prime numbers, where it is used to derive the existence of another prime.

Applications and properties

Primorials play a role in the search for prime numbers in additive arithmetic progressions. For instance,
+ 23# results in a prime, beginning a sequence of thirteen primes found by repeatedly adding 23#, and ending with. 23# is also the common difference in arithmetic progressions of fifteen and sixteen primes.
Every highly composite number is a product of primorials.
Primorials are all square-free integers, and each one has more distinct prime factors than any number smaller than it. For each primorial, the fraction is smaller than it for any lesser integer, where is the Euler totient function.
Any completely multiplicative function is defined by its values at primorials, since it is defined by its values at primes, which can be recovered by division of adjacent values.
Base systems corresponding to primorials have a lower proportion of repeating fractions than any smaller base.
Every primorial is a sparsely totient number.
The -compositorial of a composite number is the product of all composite numbers up to and including. The -compositorial is equal to the -factorial divided by the primorial. The compositorials are

Appearance

The Riemann zeta function at positive integers greater than one can be expressed by using the primorial function and Jordan's totient function :

Table of primorials