21397 has 2 divisors, whose sum is σ = 21398. Its totient is φ = 21396.

The previous prime is 21391. The next prime is 21401. The reversal of 21397 is 79312.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21316 + 81 = 146^2 + 9^2 .

It is a cyclic number.

It is not a de Polignac number, because 21397 - 2⁷ = 21269 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21391) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10698 + 10699.

It is an arithmetic number, because the mean of its divisors is an integer number (10699).

2²¹³⁹⁷ is an apocalyptic number.

It is an amenable number.

21397 is a deficient number, since it is larger than the sum of its proper divisors (1).

21397 is an equidigital number, since it uses as much as digits as its factorization.

21397 is an evil number, because the sum of its binary digits is even.

The product of its digits is 378, while the sum is 22.

The square root of 21397 is about 146.2771342350. The cubic root of 21397 is about 27.7620135738.

The spelling of 21397 in words is "twenty-one thousand, three hundred ninety-seven".