CHAPTER 8

On the history of the problem of determining the number of prime and its related functions ""and ""


8.1. An introduction to the function ""and ""

"" and "" functions which we will be acquainted with them are important functions that are used in number theory. In these discussions, we prove some of primary properties and say some information about it and we will be acquainted with Chebyshevís theorem about restriction "".

8.1.1. Attention

After this, "x" is a real and positive number and "" means natural logarithm.

8.1.2. Definition

For every positive real number "x", "" is the number of prime numbers which arenít more than "x".

8.1.3. Direct results of definition

I. With definition, it is clear:

†††††††††† (1)

That "" includes all of prime numbers "p", then it adapt in †propositional. Below relations are some of direct results of definition:

 

††††††††††††††††††††††† (2)

†††††††††† (3)

†† (4)

 

( is r-th term of prime numbers sequence) Specially:

 

††††††††††††††††††††††††††††††† (5)

 

Generally if then †then it is possible to show the sequence of prime numbers not more than "x" as follow:

 

††††††††††††††††††††††††† (6)

 

II. If "x" and "y" be two real numbers and "", then:

†††† ††††††††††††††††††††††††(7)

8.1.4. Remark

We can define "" according to this formula (7)for every x (positive or non-positive). However, because first prime number is "2", for "" we have "".

 

III. "" function is ascending with above assumption about "x" and "y":

 

††† †††††††††††††††(8)

 

We will acquaint with some of other primary properties of "" function.

8.1.5. Theorem

For "":

††††††††††††††††††† ††††††††††††††††††††††(9)

Now we start to restrict "" from bottom.

8.1.6. Theorem.

 

Always:†††††††††††††††††††††††††††††

††††††††††††††††††††††††(10)

Proof. As usual, we consider that "e" is bray centric logarithm. If "" then "". Now we consider that "" and we select strictly ascending sequence namely:

It is clear that there is a natural number like "n", so that "":

So:

Therefore, because "" function is ascending, "" in the other side, because "", according to logarithm properties, "" and.

This inequality is very weak, and information about it is so little. For example for "":

So, information that is "" although "".

8.1.7. Theorem

 

Always:

††††††† ††††††(11)

 

Proof. According to previous theorem, we consider that "". So prime numbers which arenít more than "m", are:

Therefore, "" and "" and this is equal to (11) inequality. According to (11) inequality, for example:

And this is also very little with respect to real value of "".

8.1.8. Theorem

For every natural number "n":

 

†††††††††† (12)

 

Proof. According to inequalities of theorem (8.1.7) ,"" this inequality is equal to (12) inequality.

 

8.1.9. Result

According to above theorems, always:

†††††††††††††† (13)

But donít think that research problem about measure of "" can be solved by these inequalities. In fact finding a good approximate value for "" is most complicated than it seems firstly.

According to above inequalities, for "", "" but, in fact for large values of "", "" is so smaller than what seems from (13) inequality. But:

†††††††††† (14)

 

Proving (14) equality is possible with primary instruments.

8.1.10. Lemma

For every positive real number of "" and every natural number of "k":

†††††††††† (15)

8.2. Prime numbers theorem

†Research for measuring of "" is subject of a lot of works and important resulted has been obtained in this field. Center of researches was inquiry in prime numbers tables and comparison "" with known functions. If you refer to little table:

 

x

100

200

300

400

500

600

700

800

900

1000

10000

100000

1000000

10000000

100000000

25

46

62

78

95

109

125

139

154

168

1229

9592

78498

664579

5761455

 

And calculate "", you can find that answer is between "0.9" and "1.2" with this method. For first time, relationship between "" and "" was founded. First mathematicians who searched about "" were Legendre and Gauss. Legendre (1798) said an assertion that for great value of "x":

††††††††††††††† (16)

Able described this assertion (a letter in 1832) as "the most considerable theorem in all of mathematics". Gauss had started his searches in this field apparently around the years 1792 and 1793, and published firstly in 1863. Gauss as we saw in above little table, guessed when "x" become great, the ratio of "" to "", approaches to "1" and in the other hand:

††††††††††††††††††††††††††† (17)

Also, he counted logarithmic integral of "x" as better approximate value for "" from "".

But, none of two mentioned scientists could prove their tentative assertions.

The first mathematician who calculated a verified assertion about behavior of "", was Chebyshev (1850). He proved that "" is from order of "". Exactly he proved that If "x" be great sufficiently:

††††††† (18)

Also he proved that if mentioned limit exists in (17), it is equal to "1". But relation (17) remained as an enigma so that Riemann solved that problem in 1859. His demonstration to prove this relation isnít perfect and enough. But it has necessary thought for a perfect demonstration. Finally, in (1896), "Hadamard and Poussin separately, proved relation (17) and this theorem which is famous as prime numbers theorem", is central core of prime numbers theorem. Hadamard and Poussin's demonstration is by means of complex function theorem and for many years, processes of this theorem is basic and inevitable to proving prime numbers theorem, so that in 1948, Selberg and Ardosh presented primary demonstration to this theorem and this had important effect between scientists of number theory. It is necessary to say that demonstrations are very complicated and detailed and primary description for them means its independence of complex functions theorem not in normal meaning.

What is said above was just for more information and will not be proved in this book. But we present Chebyshevís theorem about natural numbers:

†††††††††† (19)

Its Proof is possible with primary instruments. Thinking about below numbers is useful to comparison "" function with below functions:

.†††††††

8.3. The function"" or "the logarithmic integral"

"li" function is defined by this equality:

†††††† †††††††††††††††(20)

That "c" is a constant number. For abridge, we write:

††††† ††††(21)

In below table, values and are written for some values of "x".

Comparison table of "" and ""

1000

168

178

0.94

1.159

10000

1229

1246

0.98

1.132

50000

5133

5167

0.993

1.111

100000

9592

9630

0.996

1.104

500000

41538

41606

0.9983

1.090

1000000

78498

78628

0.9983

1.084

2000000

148933

149055

0.9991

1.080

5000000

348513

348638

0.9996

1.075

10000000

664579

664918

0.9994

1.071

20000000

1270607

1270905

0.9997

1.068

90000000

5216954

5217810

0.99983

1.062

100000000

5761455

5762209

0.99986

1.061

1000000000

50847534

50849235

0.99996

1.053

 

It is proved:

†††††††††† †††††††††††††††††††(22)

It is wonderful that a simple function like "" shows the irregular function "", so good (see 4th column).

In above table, always "", but this assertion is not general. In 1914, Littlewood proved that for, "" is infinitely positive or negative. According to Skewes's researches (1933 and after that) a number like "" exists that is smaller than †and.

8.3.1. Remark

†Prime numbers theorem for arithmetical progression is that† If "a" and "m" be two considered coprime natural numbers and be numbering of prime numbers in the form of "" that arenít† more than "x" then:

†††† ††††††††††††††††(23)

This theorem also, like prime numbers theorem is provable with primary way.

8.3.2. Remark

For every two real numbers "" and "" that "":

††† ††††††††††††††††††††††††††††††(24)

8.4. Meissel's formula for ""

†By presented formula Calculating "" is a tedious work. In 1870, Meissel presented a formula for calculation of "" and after long calculation, it resulted:

8.4.1. Example.

 

Calculating "":

, and Ö

 

From prime numbers table:

Therefore, according to Meissel's formula:

††††††† ††††††††††(25)

According to the prime numbers table:

,

So, according to (25):

††††††††††††††††††† ††††††††††††††††(26)

To calculate we use from proved lemma:

According to other proved lemma:

Since †and so according to before lemma:

So:

Then †and therefore:

 

 

Subsequently:

168

8.4.2. Result

By now, we have remarked some of the most important results about determining the number of prime numbers which are not more than a supposed number "N", that most important one was "Meisselís formula" by which for determining the number of prime numbers, "", all of prime numbers not more than"" and also the value of "" for every "y" so that "", should be determined before.

Despite some modification in Meisselís formula, done by Lamer who prepared it to be calculated by electronic calculators, it is not possible to apply it in scientific and practical manner and we have shown how many acknowledged parameters should be used for calculation of.

It is obvious that if "N" is a great number, calculating the "" will be impossible. In fact, we are seeking a simple relation that gives us the numbers of prime numbers, not more than "N", with an acceptable estimation. It is clear that because of the great gap between the prime numbers, it is impossible to find a certain relation, by which the number of prime numbers can be determined.