Prime numbers

(notes by Roberto Bigoni)

1. Prime numbers

As we know from our early arithmetic studies, prime numbers are the natural numbers greater than 1, which aren't the product of two other numbers both less than the number itself.

This definition implies that 1 is not a prime number, and that the only even prime number is 2.

Prime numbers are infinite, that is, the set of prime numbers does not have maximum. A proof by contradiction of this theorem is due to Euclid.

If prime numbers were not infinite, then would exist the maximum prime number M, and it would be possible to calculate the number P = 2·3·5·7·11·13·…·M, equal to the product of all the prime numbers.

Let us consider the natural number S immediately following P: S=P+1.

It is impossible that S is a prime number, because it is greater than the maximum M.

If we were to divide S for any prime number, we would have a quotient given by the product of the remaining prime numbers and the remainder 1, then S is not divisible by any of the prime numbers, so it must be prime.

We get two contradictory statements and we must therefore conclude that the primes are infinite.

Natural numbers greater than 1 that are not prime are said composite numbers.


2. The Sieve of Eratosthenes

The easiest method to get the set Pn of the primes ≤ n (n>13) is the Sieve of Eratosthenes
(sieve: a tool used for separating coarse from fine parts of loose matter):

  1. we immediately put in Pn the numbers 2, 3, 5, 7;
  2. we form the set Dn of all the odd numbers d: 7 < dn;
  3. we eliminate from Dn all the multiples of 3, 5, 7;
  4. the first of the numbers remained Dn is prime: we remove it from Dn and we put it in Pn;
  5. we eliminate from Dn all the multiples of this number;
  6. we repeat the steps 4 and 5 until Dn remains empty;
  7. at the end of the procedure Pn contains all the searched primes.



To determine if an odd number n is prime, that is, to check its primality, we can build the set Pn and check whether nPn.

This procedure, however, is unnecessarily costly since a possible divisor of n can not exceed its square root. Therefore, we should

  1. calculate the set Qn = {q1, q2, … qn } of the primes less than the square root of n;
  2. calculate the remainder of the division of n for each of the elements of Qn starting from the first;
  3. if we get a remainder equal to 0, n is not prime, and the procedure ends;
  4. otherwise n is prime.

But even with this improvement, if n is very large, the sieve becomes impractical because it may require too much computation time. Direct primality tests, that is such that do not require the previous calculation of a huge number of primes, were searched and are searched even today.


3. Fermat's factorization method.

One of the first direct primality test is due to P. de Fermat.

Given the odd number n, the Fermat's method is based on the research of two numbers h and k such that


If n is a perfect square, the solution is immediate because in this case n is equal to the product of its square root for itself and, of course, n is not prime.

If n is not too big it is convenient to apply the Sieve of Eratosthenes.

Otherwise we can always express hk as a difference of two squares:


then, with


we get


The problem is solved if we find a number a such that a2-n is a perfect square.

For this purpose we proceed by trials:

Given a e b, we have


If a-b = 1, n is prime; otherwise the two factors a-b and a-b (that is h e k) are different and less than n, then n is composite.


4. Perfect squares

Fermat's method requires that we repeatedly check whether a natural number n is a perfect square. It is not possible to directly determine this property of n. However, in many cases we can eliminate quickly enough this possibility, noting that the quadratic residues modulo m, for a fixed number m, are quite a few.

For example, the quadratic residues modulo 8 for the squares of the first 100 natural numbers, using WolframAlpha, are


Even without a formal proof, it can safely be conjectured that by calculating the remainder of the division of a natural number by 8, if the remainder is equal to 0, 1, 4, the number is not a perfect square.

A further example is provided by the set of the quadratic residues modulo 9.


Even in this case we can safely conjecture that by calculating the remainder of the division of a number by 9, if the remainder is equal to 0, 1, 4, 7, the number is not a perfect square.

But to pass a test of this kind is necessary but not sufficient condition to conclude that n is a perfect square. It is a useful control to narrow down the search for possible perfect squares, but not conclusive. To successfully identify a perfect square we inevitably need a more expensive procedure, like the following bisection algorithm.

  1. We consider the sequence of the natural numbers qi that, starting from 4, is formed by numbers such that each of them is the square of the previous number.


  2. If n coincides with one of these numbers, n is a perfect square, and the procedure ends.
  3. Otherwise even its square root √n does not coincide with a qi and then it is comprised between two of these terms:
  4. we let
  5. we compare n with the square of the mean value m: Eqn009.gif


Let n = 40401

We have 40401≡1 (mod 8) and 40401≡0 (mod 9), so n passes the two proposed preliminary tests. Now we apply the bisection algorithm:

step 1Eqn010.gif
step 2Eqn011.gif
step 3Eqn012.gif
step 4Eqn013.gif
step 5Eqn014.gif
step 6Eqn015.gif
step 7Eqn016.gif
endn is a perfect square; its square root is 201


5. Prime factorization of a number

A natural number n, greater than 1, either is prime or is composite. If n is composite, by the Fermat method, n can be expressed as the product of two natural numbers h and k less than n. In their turn, each of the numbers h and k either is prime or is composite. If they are both prime, their product is the decomposition of n into prime factors product (or factorization of n). Otherwise, we can apply the Fermat's method to the composite factors and repeat the process until we have only prime factors. If at the end of the procedure a factor appears several times, the product of the identical factors is replaced by a power and the product of these powers (with exponent ≥ 1) is the unique decomposition of n into product of powers of prime factors.

The Fermat's method works satisfactorily for not too big numbers or, even for big numbers, if the two factors h and k are both next to √n. Otherwise, it may take an impractical computing time even on the fastest systems, particulary when the number n that we try to factorize is a big prime number, for which the sought factors h and k are n itself and 1, then very distant from √n.

Alternatively we could use the Sieve of Eratosthenes.

  1. We calculate the set Qn = {q1, q2, … qn } of the primes less than the square root of n
  2. If no qi divides n, n is prime.
  3. If n is divisible by a qi, we have identified a factor of the decomposition; we perform the division and we check whether the quotient is still divisible by qi.
    If yes, we repeat the division until we get a quotient not divisible by qi. qi appears in the factorization with exponent equal to the number of divisions.
  4. We apply again this procedure to last obtained quotient by checking whether it is divisible by the remaining elements of Qn.

However, if n is very big, even √n is big and is also huge the number of the elements of Qn whose calculation can require an impratical time, especially if n is prime, because it is necessary to perform the the division of n by each qi before coming to the conclusion that n is prime.

So before we try factoring large numbers, we do well to ensure with more efficient methods that n is not a prime number. In general these methods are probabilistic, that is such as not to produce the mathematical certainty of the primality of n, but to guarantee it with such a high degree of probability to be fully reliable in many operating situations.


6. The Fermat primality test

Historically, the first of these methods is even due to Fermat and is based on a theorem enunciated by Fermat himself, but subsequently demonstrated by L. Euler, known as Fermat's little theorem.

If p is a prime number, then, for any integer n, Eqn017.gif

where npn(mod p) means that the divisions by p of np and n give equal remainders. We say that np and n are congruent modulo p.

For example: given n=10 and p=3, we have:

A demonstration of the theorem may be obtained by induction.

First of all we observe that, if p is prime,


In fact, if we expand the power of the binomial, we get




The theorem holds for n=0 and n=1:


If the theorem holds for n


from the equation (1) we get


Then, by induction, the theorem holds for all n.

In particular, if n has no common divisors with p, that is it is coprime with respect to p, we have


The theorem states a necessary, but not sufficient condition, that is any prime p verify the stated congruence, but this congruence may be verified also by some composite number c. These numbers are said Fermat pseudoprimes with respect to n. In particular, the pseudoprimes c with respect to any n coprime with respect to c are said Carmichael numbers.

So, if a number q does not verify the theorem with respect to a coprime n, we can say that q is composite, but if it verifies the congruence, we can not say that it is prime, but only that it is a probable prime.

If we try with many random values of n with always positive results, we can operationally assume q as a prime number.


1. Let q=41.

The probability that q is not a prime number is quite small. Indeed 41 is prime, as we can check with the Sieve of Eratosthenes.

2. Let q=91.

3. Let q=561.

If we apply the Fermat's factorization method, we get 561 = 17 · 33. So 561 is composite.

Now we try the Fermat primality test:



5. Miller-Rabin primality test

The Fermat primality test, even with several trials, can fail, that is it can lead us to consider as prime a number that instead is composite. We can reduce this risk and estimate the probability of getting the correct answer, noting that a prime number q>2 must be odd and then q-1 must be even. Moreover, every even number can be decomposed into the product of an odd number d and a power s of 2.


By Fermat's theorem, if q is prime, and n is coprime with respect to q, Eqn032.gif, we have




If this root is ≡ 1(mod q), then the next will be ≡ ±1 (mod q), and so on.

If all the roots are ≡ 1 (mod q), so even nd≡1 (mod q), q passes the test and is probably prime.

If the first root not ≡ 1 (mod q) is = -1 (mod q), q passes the test and is probably prime.

Otherwise q is composite.

In conclusion, to determine whether q is probably prime:

The probability that a composite number q passes the test is at most ¼. Therefore, by repeating the test with other values of n, the probability that q can pass them all decreases exponentially.


1. As we have seen the number q=561, is a Fermat pseudoprime, therefore, while being composite, it passes the Fermat test.

We decompose 560 into the product between an odd number and a power of 2: 560=35·24

With n=2, we have 235=34359738368; 235≡ 263 (mod 561)

2280≡1 (mod 561)

2140≡67 (mod 561)

So q is composite.


2. Let q=601

With n=2, we have
2600≡1 (mod 601): the number passes the Fermat test.


With n=2 we have

275=37778931862957161709568; 275≡ 1 (mod 601): 601 is a probable prime.


3. Let q=401

With n=2, we have
2400≡1 (mod 401): 401 passes the Fermat test.


With n=2 we have

225=33554432; 275≡356 (mod 401)

2200=1606938044258990275541962092341162602522202993782792835301376; 2200≡ 1 (mod 401);

2100=1267650600228229401496703205376; 2100≡ -1 (mod 401): 401 is a probable prime.



6. Calculator




7. Very big numbers.

The applications of this page are implemented in JS and, while enhanced by the BigInt library of Leemon Baird, they are inevitably too slow to handle very big numbers. If we have to decompose in very large numbers, we can try to use WolframAlpha.

It should however be noted that, even using the most advanced software and specially designed hardware, the factorization of a very big number may require unworkable computational time and this fact is the basis of the most efficicient encryption methods, such as RSA.

last revision 26/06/2016