Adding to 21 its reverse (12), we get a palindrome (33).
Multipling 21 by its reverse (12), we get a palindrome (252).
It is the 7-th Fibonacci number F7.
21 is nontrivially palindromic in base 2, base 4 and base 6.
21 is digitally balanced in base 3, because in such base it contains all the possibile digits an equal number of times.
21 is an esthetic number in base 2, base 3, base 9 and base 10, because in such bases its adjacent digits differ by 1.
It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also a brilliant number, because the two primes have the same length.
It is the 5-th Motzkin number.
21 is an idoneal number.
It is the 6-th Jacobsthal number.
It is a D-number.
It is an alternating number because its digits alternate between even and odd.
It is the 5-th Hogben number.
It is a Duffinian number.
21 is an undulating number in base 2.
21 is strictly pandigital in base 3.
It is a Curzon number.
21 is a lucky number.
21 is a nontrivial repdigit in base 4 and base 6.
It is a plaindrome in base 4, base 6, base 8, base 9, base 11, base 12, base 13, base 14, base 15 and base 16.
It is a nialpdrome in base 3, base 4, base 5, base 6, base 7 and base 10.
It is a zygodrome in base 4 and base 6.
It is a congruent number.
It is a panconsummate number.
It is a 2-hyperperfect number.
It is an amenable number.
21 is an equidigital number, since it uses as much as digits as its factorization.
21 is an odious number, because the sum of its binary digits is odd.
The sum of its prime factors is 10.
The square root of 21 is about 4.5825756950. The cubic root of 21 is about 2.7589241764.