# Is there any way to find the product of prime numbers?

1. Sep 4, 2011

### l-1j-cho

According to the prime number theorem, the number of prime numbers that are less than N is approximately N\ln(N) for a sufficiently large N. But can we find the product of prime numbers that are less than N?
(For example, N=20 then 2x3x5x7x11x13x17x19 although I think 20 isn't large enough haha)

2. Sep 4, 2011

### HallsofIvy

Staff Emeritus
Of course there is- multiply them!

3. Sep 4, 2011

### robert2734

If you are looking for a list of semiprimes, it exists. http://oeis.org/A001358/b001358.txt

If you are looking for a formula pik(n)=(ln(n)/n)*((n^k)/k!). Where k is the number of prime factors not necessarily unique.

4. Sep 5, 2011

### RamaWolf

Definition: the producht of the first n primes is called primorial and is writtn as n#

Theorem: n# < $4^{n}$

Definition: $\vartheta(x) := \Sigma_{p_{i}<=x} Log(p_{i})$ (Chebychev)

Theorem: $\vartheta(x)$ ~ x for x -> $\infty$

5. Sep 5, 2011

### RamaWolf

For the second Theorem above from my 'numerical department':

i |p$_{i}$ |$\vartheta (p_{i})$ |$\vartheta (p_{i})/p_{i}$

5 | 11 | 7.745 | 0.704091
26 | 101 | 88.344 | 0.874688
169 | 1009 | 963.162 | 0.954571
1230 | 10007 | 9905.202 | 0.989827
9593 | 100003 | 99696.902 | 0.996939

6. Sep 5, 2011

### l-1j-cho

could you let me know the name of the first theorem so i can google it?

7. Sep 5, 2011

### RamaWolf

I think, there is no sprecific name for it; but go to:

//en.wikipedia.org/wiki/Proof_of_Bertrand%27s_postulate

and there look for Lemma 4