# On the upper bound of number-theoretic function

## Full text

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f

### The high school attached to Tsinghua University

Abstract: The problem of the existence of infinitely many prime values of a number-theoretic function

### f x ( )

has been one of the most important topics in Number Theory. Note that if

### f x ( )

represents infinitely many primes, then we can get this necessary condition: for any positive integer

### h

, there exists a positive integer

such that

and

### f k ( ) 1 >

. Naturally, we are interested in the number-theoretic functions

### f x ( )

that satisfy the aforementioned necessary condition. Thus, there must exist the least positive integer such that and . Denote this least positive integer by . In this paper, we mainly

focus on three famous number-theoretic functions:

f

2x

,

x

### − 1

and , proving they satisfy the aforementioned necessary condition respectively. Furthermore, we approximately estimate the upper bound of respectively, and obtain some interesting results.

2

( )

( )

( )

f x m x l x

Let

### f x ( )

be a number-theoretic function. Whether

### f x ( )

represents infinitely many primes has always been a problem that attracts great interests among many famous mathematicians. As early as 2000 years ago, Euclid has proved that

### f x ( ) = x

represents infinitely many primes. In 1837, Dirichlet proved that

### f x ( ) = ax b +

also represents infinitely many prime values, where

and are integers with , either or

### ab ( , ) 1 a b = a > 0, b ≠ 0 a = 1, b = 0

. By proving this, he completely solved this problem in the case of linear polynomial with integral coefficients. Apart from this case of linear polynomial, however, the problem becomes complex and there is still no complete solution by now. For example, whether functions such as

, and

2x

x

2

### + 1

represent infinitely many primes has not been verified until today. Actually, these functions are respectively related to the problem of Fermat number, Mersenne number and the first conjecture of Landau. Readers can refer to [6] for some other information. [6] is a paper that summaries the research history from Euclid to Green-Tao Theorem, studies the infinitude of some special kinds of prime and brings up some interesting questions. Our paper is just based on one of these questions. I thank Doctor Shaohua Zhang here for suggesting this interesting topic to us.

Note that if a number-theoretic function

### f x ( )

represents infinitely many primes, we can get the

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following necessary condition: for every positive integer , there exists a positive integer , such that , . Consequently, we are interested in functions that satisfy this necessary condition. Thus, if

### f x

satisfies this condition, there exists the smallest integer such that , . Denote by . In this paper, we mainly focus on the three aforementioned functions, proving that they all satisfy this necessary condition, and approximately estimate the upper bound of . Here are the results obtained.

f

( )

( )

( )

f x m x l x

Theorem 1 If

x

### − 1

, then for every positive integer , there exists a least positive integer such that and . In addition, there exists a constant such that for every

, ( )

2

m x

### n = Fh < h

.

Theorem 2 If , then for every positive integer , there exists a least positive integer such that for every

2x

,

f x( )

2

.

Theorem 3 If

2

### + 1

, then for every positive intege , there exists a least positive integer , such that . In addition, for every

r

,

, ( )

l x

.

### Ⅱ． Proof of the theorems

In this paper, we denote the gre

j

### 1

atest common divisor of

and

by

### ( , ) a b

. Lemma 1[5] For every positive integer

,

i

j

( ,i )

### −

is a prime factor of p

.

f

,

Lemma 2[4,5] Let

### p

be an odd prime. I .

m Lemm every positive integer ,

Remark 1: Fro a 2, we can get that for

( )

m x

### n = Fh ≤ ⎡ h + ⎤

Lemma 3[1] For integer

### ⎣ ⎦

.We give a stronger bound in the proof of Theorem 1.

,

p x

## ∑

, where

### logp

represents the nature log m a 4[2] For every integer , there exists at least one prime in

of

.

Le m

. Lemma 5[3] For

### k > 1

, the smallest prime factor of

2k

is no less than

k+2

### + 1

.

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Proof of theorem 1: Remark 1 actually indicates that the first half of theorem 1 is true, so we only

eed to prove that there exists a constant such that for

n

every

, ( )

2

m x

here. Let

rbe the

thprime. If

pr

### = h

3/ 4, there is only

one solution

r

,

, and ( )

m x

2

. Thus, w

e can let pr

4 pr1

3

### 2 < < 2

+ .

If there exists an

,

, such that

pi

, then

pi

pr

### < h

3 / 4. As a result,

( ) 2

m x i r

every

,

pi

### − 1, ) h > 1

. This means we only have to consider the case that for . From Lemma 1 we know that for every ,

i pj

p

Lemm

is no less then

i i

i=

proving tha

r

r

p

i

+

## ∏

, thus finishing our proof. But to prove this was then beyond my ability. Thank to my teacher Jinsong Li, who introduced some beautiful results of analytic number theory to me, especially on the bound of Chebyshev function

### (2 1, 2 1 = 1

. And from a 2, we know that for every

### i ≥ 2

, the smallest prime factor

of

. Hence,

r

. When

o

i=1

p x

pi

2

1 r

i i

=

4pr+1/ 3 can o a

t 4 1/ 3

## ∑

, which shed new light on my proving this problem. From Lemma 3, we know that for

r

,

1

i

r

i r

≤ ≤

. Because

, and

wh

from Lemma 4:

en

r

, 1

r

r

+

, we have that 1

1

≤ ≤

i

r r

i r

+

## ∑

. Thus,

for

r

### ≥ 487381

, 4 1/ 3. Because 487381 is a prime, let , then for every

1

r

r

p i i

+

=

## ∏ > C=2

1949524/3

, ( )

2

m x

### n = Fh < h

.

Remark 2: we can prove that it is not easy to strengthen

### 3

, for instance, to

.

F

get th

rom [1], we can

at 1

1

i r r

i r

r+

≤ ≤

## ∑

. This result increase the

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difficulty of strengthening this bound.

From the proof above we also know tha p i

i

t 3 1/ 2

1

n

n

+

=

### > ∏

. Hence, for we have this

result:

n

n

n

n

n1

n

p p p p p

i

+ + +

### > > > ∏ > >

. This inequality again demonstrates the

n

### ≥

,

1 1

2 3 / 2 3 / 2 4 / 3

i=1

famous result that for every

,

### ( , 2 ) xx

contains at lea .

Besides, our inequality strengthens the inequality in [4, pp389].

Proof of theorem 2: When , we can let

st one prime

3 / 2

pn

1

n

2pn

1

n

2

, and that e sm

[log (2 1)] 1

2 h

− −

### +

From Lemma 5, we know th allest divisor of

.

### f k

is greater than . Hence we

. Therefor f x

have

[log (2 1)] 1

2 h− −

e, when

,

( )

2

. e that t

rem e

prove that when

When

### 1 ≤ ≤ h 5

, we can prov he first half is

Proof of theorem 3: The proof of the first half of the theo is obvious, so w only need to true directly.

, ( )

l x

. When

is odd, ( )

l x

. When is even,

consider t

### h

we first he case when

. Let

, where

### t

is a positive integer.

Because

2

, we have ( )

l x

. If

, because

, we let

, where

### t

is a positive integer. Since

2

, ( )

l x

### = ≤ <

.

hav hed verifying this.

to the smallest prime divisor of Thus we have proved theorem 3.

### Ⅲ． Some related questions

Question 1: In the proof of Theorem 1, is obviously a very approximate bound.

By observation and computation, howev re tha is very likely to be 84, yet we e not finis

Question 2: We don’t know how to strengthen the result in Theorem 2, but we know it is related Fermat number. Up to now, however, Lemma 5 has not been improved. Besides, it is easy to prove tha cannot be always less than

Question 3: By calculating, we conjecture that for

1949524/3

### 2 C =

er, we conjectu t

t

f x( )

2 2

.

,

l x( )

### ( ) h < h − 1

. We will nerally, Doc

k

consider this problem further. More ge tor Shaohua Zhang pointed out that if

k

"

1

### + a

0 represents infinitely many primes, there might be

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( )

l x k for every sufficiently large

### h

. He also told me it is interesting to study the primality of

,

k

f

f ,

f

,

f

### ( !))

. For example, for every positive integer

,

( )

m x

is a prime while

### ))

is not ays one; and

### )

are not

always prime. As for

m x( )

alw

l x( )

l x( )

f

### f F h

, we don’t know whether

and

### l Fh

are always primes now. However,

( )

m x l x( )

we know that

f x( )

### f Fh

is not always a prime.

, I want to first thank Professor Shing-Tung Yau for offering middle school student s va further into math. Although the result I’ve

obtained is atic study. It is an experience completely

from tests in school, nderstood both the hard d hap esearch.

ant to thank my e who helped me proofread pa e of the data, adjust the format, and ccess to many papers and ber the ding

In the end of my paper

s thi luable opportunity

a small one, it is my first try in mathem

different in which I u ship an piness of r

Here I also w mother. It is sh the per, test som

get a books of num ory. By rea

these, I deeply appreciated the beauty of Number Theory.

of exploring

### References

[1] Rosser J. Barkley, Schoenfeld Lowell, Sharper bounds for the Chebyshev functions

and

### ψ ( ) x

, Math. Comp., 29, pp243-269, 1975

[2] H. Rohrbach and J. Weis, Zum finiten Fall des Bertrandschen Postulats, J. Reine Angew. Math.

214/215, pp432-440, 1964 [3

[4

] 柯召，孙琦，初等数论100例，上海教育出版社，1980 ] 潘承洞，潘承彪，初等数论，北京大学出版社，1992

， 数论讲义， 高等教育出版社， 1987

Zhang, On the infinitude of some special kinds of primes, [5] 柯召， 孙琦

[6] Shao-hua

(http://arxiv.org/abs/0905.1655)

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