20989981 has 2 divisors, whose sum is σ = 20989982. Its totient is φ = 20989980.

The previous prime is 20989979. The next prime is 20989987. The reversal of 20989981 is 18998902.

It is a happy number.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 17222500 + 3767481 = 4150^2 + 1941^2 .

It is a cyclic number.

It is not a de Polignac number, because 20989981 - 2¹ = 20989979 is a prime.

It is a super-2 number, since 2×20989981² = 881158604760722, which contains 22 as substring.

Together with 20989979, it forms a pair of twin primes.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (20989987) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10494990 + 10494991.

It is an arithmetic number, because the mean of its divisors is an integer number (10494991).

Almost surely, 2^20989981 is an apocalyptic number.

It is an amenable number.

20989981 is a deficient number, since it is larger than the sum of its proper divisors (1).

20989981 is an equidigital number, since it uses as much as digits as its factorization.

20989981 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 93312, while the sum is 46.

The square root of 20989981 is about 4581.4824020179. The cubic root of 20989981 is about 275.8485350057.

The spelling of 20989981 in words is "twenty million, nine hundred eighty-nine thousand, nine hundred eighty-one".