21651 has 8 divisors (see below), whose sum is σ = 33024. Its totient is φ = 12360.

The previous prime is 21649. The next prime is 21661. The reversal of 21651 is 15612.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 21651 - 2¹ = 21649 is a prime.

It is a Smith number, since the sum of its digits (15) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a self number, because there is not a number n which added to its sum of digits gives 21651.

It is not an unprimeable number, because it can be changed into a prime (21601) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 495 + ... + 536.

It is an arithmetic number, because the mean of its divisors is an integer number (4128).

2²¹⁶⁵¹ is an apocalyptic number.

21651 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

21651 is a deficient number, since it is larger than the sum of its proper divisors (11373).

21651 is a wasteful number, since it uses less digits than its factorization.

21651 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1041.

The product of its digits is 60, while the sum is 15.

The square root of 21651 is about 147.1427877947. The cubic root of 21651 is about 27.8714343874.

It can be divided in two parts, 2 and 1651, that added together give a triangular number (1653 = T₅₇).

The spelling of 21651 in words is "twenty-one thousand, six hundred fifty-one".