Number
21
21 is a composite number, odd, a calendar year.
Properties
- Parity
- Odd
- Digit count
- 2
- Digit sum
- 3
- Digital root
- 3
- Palindrome
- No
- Divisor count
- 4
- σ(n) — sum of divisors
- 32
Primality
Prime factorization: 3 × 7
Divisors & multiples
Aliquot sum (sum of proper divisors):
11
Representations
- In words
- twenty-one
- Ordinal
- 21st
- Roman numeral
- XXI
- Binary
- 10101
- Octal
- 25
- Hexadecimal
- 15
Also seen as
ASCII character
As an ASCII codepoint, 21 is control character (0x15). ASCII control character.
Network port
TCP/UDP port 21 is the well-known port for FTP — File Transfer Protocol — control channel.
Hex color
#000015
RGB(0, 0, 21)
IPv4 address
As an unsigned 32-bit integer, this is the IPv4 address 0.0.0.21.