21649 has 2 divisors, whose sum is σ = 21650. Its totient is φ = 21648.

The previous prime is 21647. The next prime is 21661. The reversal of 21649 is 94612.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 17424 + 4225 = 132^2 + 65^2 .

It is a cyclic number.

It is not a de Polignac number, because 21649 - 2¹ = 21647 is a prime.

Together with 21647, it forms a pair of twin primes.

It is a nialpdrome in base 8.

It is not a weakly prime, because it can be changed into another prime (21647) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10824 + 10825.

It is an arithmetic number, because the mean of its divisors is an integer number (10825).

2²¹⁶⁴⁹ is an apocalyptic number.

It is an amenable number.

21649 is a deficient number, since it is larger than the sum of its proper divisors (1).

21649 is an equidigital number, since it uses as much as digits as its factorization.

21649 is an evil number, because the sum of its binary digits is even.

The product of its digits is 432, while the sum is 22.

The square root of 21649 is about 147.1359915181. The cubic root of 21649 is about 27.8705761579.

The spelling of 21649 in words is "twenty-one thousand, six hundred forty-nine".