21689981 has 2 divisors, whose sum is σ = 21689982. Its totient is φ = 21689980.

The previous prime is 21689971. The next prime is 21690013. The reversal of 21689981 is 18998612.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 11868025 + 9821956 = 3445^2 + 3134^2 .

It is a cyclic number.

It is not a de Polignac number, because 21689981 - 2⁶ = 21689917 is a prime.

It is a super-2 number, since 2×21689981² = 940910551560722, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21689971) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10844990 + 10844991.

It is an arithmetic number, because the mean of its divisors is an integer number (10844991).

Almost surely, 2^21689981 is an apocalyptic number.

It is an amenable number.

21689981 is a deficient number, since it is larger than the sum of its proper divisors (1).

21689981 is an equidigital number, since it uses as much as digits as its factorization.

21689981 is an evil number, because the sum of its binary digits is even.

The product of its digits is 62208, while the sum is 44.

The square root of 21689981 is about 4657.2503690482. The cubic root of 21689981 is about 278.8815116739.

The spelling of 21689981 in words is "twenty-one million, six hundred eighty-nine thousand, nine hundred eighty-one".