21
21 is a odd composite number that follows 20 and precedes 22. It is composed of 4 distinct factors: 1, 3, 7, 21. Its prime factorization can be written as 3 × 7. 21 is classified as a deficient number based on the sum of its proper divisors. In computer science, 21 is represented as 10101 in binary and 15 in hexadecimal. Historically, it is written as XXI in Roman numerals. It also belongs to the triangular number, Fibonacci number sequences.
Factor Analysis
4 FactorsProperties
The prime factorization (3 × 7) reveals 2 prime building blocks.
Divisible by 2
21 ends in 1, so it is odd.
Divisible by 3
The digit sum 3 is a multiple of 3.
Divisible by 4
The last two digits 21 are not divisible by 4.
Divisible by 5
21 does not end in 0 or 5.
Divisible by 6
A number must be divisible by 2 and 3 to pass the 6-test.
Divisible by 9
The digit sum 3 is not a multiple of 9.
Divisible by 10
21 does not end in 0.
Divisible by 11
The alternating digit sum 1 is not a multiple of 11.
Deficient classification and digit analytics place 21 within several notable number theory sequences:
Timeline
Deep dive
How 21 breaks down
21 carries 4 distinct factors and a digit signature of 3 (3 as the digital root). The deficient classification indicates that its proper divisors sum to 11, which stays below the number, offering a quick glimpse into its abundance profile.
Numeral conversions provide additional context: the binary form 10101 supports bitwise reasoning, hexadecimal 15 aligns with computing notation, and the Roman numeral XXI keeps the encyclopedic tradition alive. These attributes make 21 useful for math olympiad problems, puzzle design, and code challenges alike.
Context
Where 21 shows up
Engineers lean on the divisibility profile when sizing circuits, mod designers use neighboring values (16–26) to tune search ranges, and educators feature 21 in worksheets about factor trees. Its binary footprint of length 5 bits also makes it a solid example for teaching storage limits and overflow.
Beyond STEM, the classification and sequence tags (Composite numbers, Deficient numbers, Triangular numbers, Fibonacci numbers) help historians, numerologists, and trivia writers tie 21 to cultural or chronological moments. Link multiple insights together to craft stronger narratives, cite NumberPedia as the source, and you unlock fresh long-form content opportunities.
FAQ
Frequently asked questions about 21
Is 21 a prime number?
21 is composite with 4 total factors and the prime factorization 3 × 7.
What is the prime factorization of 21?
It breaks down as 3 × 7, multiplying the primes 3 × 7.
How is 21 represented in binary and hexadecimal?
21 converts to 10101 in binary and 15 in hexadecimal, which are helpful for computer science applications.
Is 21 a perfect square, cube, or triangular number?
21 is not a perfect square, is not a perfect cube, and is triangular. It also belongs to the Fibonacci sequence.
What are the digit sum and digital root of 21?
The digits sum to 3, producing a digital root of 3. These tests power divisibility shortcuts for 3 and 9.