21647 has 2 divisors, whose sum is σ = 21648. Its totient is φ = 21646.

The previous prime is 21617. The next prime is 21649. The reversal of 21647 is 74612.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21647 - 2⁸ = 21391 is a prime.

Together with 21649, it forms a pair of twin primes.

It is a Chen prime.

It is a nialpdrome in base 15.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21649) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10823 + 10824.

It is an arithmetic number, because the mean of its divisors is an integer number (10824).

2²¹⁶⁴⁷ is an apocalyptic number.

21647 is a deficient number, since it is larger than the sum of its proper divisors (1).

21647 is an equidigital number, since it uses as much as digits as its factorization.

21647 is an evil number, because the sum of its binary digits is even.

The product of its digits is 336, while the sum is 20.

The square root of 21647 is about 147.1291949275. The cubic root of 21647 is about 27.8697178754.

The spelling of 21647 in words is "twenty-one thousand, six hundred forty-seven".