A number is classified as a prime if it is greater than one and is divisible only by one and by itself. Only natural numbers are classified as primes. Before knowing more about the Prime number, it is important to remember some divisibility rules, which help in identifying which numbers are not prime.
Divisibility by 2: every even number is divisible by 2. Even numbers are those ending in 0, 2, 4, 6 and 8.
Divisibility by 3: a number is divisible by 3 if the sum of its digits gives a number divisible by 3.
Divisibility by 4: a number is divisible by 4 if it is divisible twice by 2, or if its last two digits are divisible by 4.
Divisibility by 5: every number ending in 0 or 5 is divisible by five.
Divisibility by 6: if a number is even and also divisible by 3, it will be divisible by 6.
Divisibility by 7: a number is divisible by 7 if the difference between double the last digit and the remainder of the number results in a multiple of 7.
These are the main rules of divisibility. To find each prime number less than 100, we use the “
Sieve of Eratosthenes”. In the following table, we will cancel the non-prime numbers in this order:-
The number 1 will be out because, by the initial condition, the prime numbers are greater than one (it will be highlighted from black);
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Numbers ending in 0, 2, 4, 6 and 8 will be out because they are divisible by two (they will be highlighted Red);
Numbers ending in 5 will be out because they are divisible by 5 (they will be highlighted from blue). Numbers ending in zero have already been cut;
Numbers whose sum of digits is 3 will be out because they are divisible by three (will be highlighted from Orange);
Numbers that are divisible by 7 will be removed as well (will be highlighted from green)
The numbers highlighted in yellow are those that are only divisible by 1 and by themselves, that is, they do not meet any of the divisibility criteria mentioned above. Therefore, by the "Riddle of Eratosthenes", the numbers 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47.53, 59, 61, 67, 71, 73, 79, 83, 89 and 97 they are the only prime numbers less than 100.
In the initial text image, there are several prime numbers between 100 and 1000. Today a large number of prime numbers are known, but it is not known which is the largest existing prime number. This is one of the great math puzzles that will make your puzzler rich. There is a millionaire prize for the one who discovers the biggest of the prime numbers.
By Amanda Gonçalves
Graduated in Mathematics
Would you like to reference this text in a school or academic work? Look:
RIBEIRO, Amanda Gonçalves. "What is a prime number?"; Brazil School. Available in: https://brasilescola.uol.com.br/o-que-e/matematica/o-que-e-numero-primo.htm. Accessed on June 27, 2021.
