20821981 has 2 divisors, whose sum is σ = 20821982. Its totient is φ = 20821980.

The previous prime is 20821963. The next prime is 20822017. The reversal of 20821981 is 18912802.

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., 10956100 + 9865881 = 3310^2 + 3141^2 .

It is a cyclic number.

It is not a de Polignac number, because 20821981 - 2⁵ = 20821949 is a prime.

It is a super-2 number, since 2×20821981² = 867109785528722, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (20821081) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10410990 + 10410991.

It is an arithmetic number, because the mean of its divisors is an integer number (10410991).

Almost surely, 2^20821981 is an apocalyptic number.

It is an amenable number.

20821981 is a deficient number, since it is larger than the sum of its proper divisors (1).

20821981 is an equidigital number, since it uses as much as digits as its factorization.

20821981 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 2304, while the sum is 31.

The square root of 20821981 is about 4563.1108906096. The cubic root of 20821981 is about 275.1106155553.

The spelling of 20821981 in words is "twenty million, eight hundred twenty-one thousand, nine hundred eighty-one".