21951 has 10 divisors (see below), whose sum is σ = 32912. Its totient is φ = 14580.

The previous prime is 21943. The next prime is 21961. The reversal of 21951 is 15912.

It is a Cunningham number, because it is equal to 28³-1.

It is not a de Polignac number, because 21951 - 2³ = 21943 is a prime.

It is a Duffinian number.

It is a plaindrome in base 4, base 14 and base 16.

It is a nialpdrome in base 9.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21911) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 9 ways as a sum of consecutive naturals, for example, 55 + ... + 216.

2²¹⁹⁵¹ is an apocalyptic number.

21951 is a deficient number, since it is larger than the sum of its proper divisors (10961).

21951 is an equidigital number, since it uses as much as digits as its factorization.

21951 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 283 (or 274 counting only the distinct ones).

The product of its digits is 90, while the sum is 18.

The square root of 21951 is about 148.1586986984. The cubic root of 21951 is about 27.9995748235. Note that the first 3 decimals are identical.

It can be divided in two parts, 2 and 1951, that added together give a triangular number (1953 = T₆₂).

The spelling of 21951 in words is "twenty-one thousand, nine hundred fifty-one".