21997 has 2 divisors, whose sum is σ = 21998. Its totient is φ = 21996.

The previous prime is 21991. The next prime is 22003. The reversal of 21997 is 79912.

It is a balanced prime because it is at equal distance from previous prime (21991) and next prime (22003).

It can be written as a sum of positive squares in only one way, i.e., 19881 + 2116 = 141^2 + 46^2 .

21997 is a truncatable prime.

It is a cyclic number.

It is not a de Polignac number, because 21997 - 2¹¹ = 19949 is a prime.

It is a Chen prime.

It is a plaindrome in base 11.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21991) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10998 + 10999.

It is an arithmetic number, because the mean of its divisors is an integer number (10999).

2²¹⁹⁹⁷ is an apocalyptic number.

It is an amenable number.

21997 is a deficient number, since it is larger than the sum of its proper divisors (1).

21997 is an equidigital number, since it uses as much as digits as its factorization.

21997 is an evil number, because the sum of its binary digits is even.

The product of its digits is 1134, while the sum is 28.

The square root of 21997 is about 148.3138563992. The cubic root of 21997 is about 28.0191195944.

The spelling of 21997 in words is "twenty-one thousand, nine hundred ninety-seven".