21is an odd number,as it is not divisible by 2
The factors for 21 are all the numbers between -21 and 21 , which divide 21 without leaving any remainder. Since 21 divided by -21 is an integer, -21 is a factor of 21 .
Since 21 divided by -21 is a whole number, -21 is a factor of 21
Since 21 divided by -7 is a whole number, -7 is a factor of 21
Since 21 divided by -3 is a whole number, -3 is a factor of 21
Since 21 divided by -1 is a whole number, -1 is a factor of 21
Since 21 divided by 1 is a whole number, 1 is a factor of 21
Since 21 divided by 3 is a whole number, 3 is a factor of 21
Since 21 divided by 7 is a whole number, 7 is a factor of 21
Multiples of 21 are all integers divisible by 21 , i.e. the remainder of the full division by 21 is zero. There are infinite multiples of 21. The smallest multiples of 21 are:
0 : in fact, 0 is divisible by any integer, so it is also a multiple of 21 since 0 × 21 = 0
21 : in fact, 21 is a multiple of itself, since 21 is divisible by 21 (it was 21 / 21 = 1, so the rest of this division is zero)
It is possible to determine using mathematical techniques whether an integer is prime or not.
for 21, the answer is: No, 21 is not a prime number.
To know the primality of an integer, we can use several algorithms. The most naive is to try all divisors below the number you want to know if it is prime (in our case 21). We can already eliminate even numbers bigger than 2 (then 4 , 6 , 8 ...). Besides, we can stop at the square root of the number in question (here 4.583 ). Historically, the Eratosthenes screen (which dates back to Antiquity) uses this technique relatively effectively.
More modern techniques include the Atkin screen, probabilistic tests, or the cyclotomic test.
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