20621981 has 2 divisors, whose sum is σ = 20621982. Its totient is φ = 20621980.

The previous prime is 20621977. The next prime is 20621999. The reversal of 20621981 is 18912602.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 10956100 + 9665881 = 3310^2 + 3109^2 .

It is a cyclic number.

It is not a de Polignac number, because 20621981 - 2² = 20621977 is a prime.

It is a super-2 number, since 2×20621981² = 850532200728722, which contains 22 as substring.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (20621921) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10310990 + 10310991.

It is an arithmetic number, because the mean of its divisors is an integer number (10310991).

Almost surely, 2^20621981 is an apocalyptic number.

It is an amenable number.

20621981 is a deficient number, since it is larger than the sum of its proper divisors (1).

20621981 is an equidigital number, since it uses as much as digits as its factorization.

20621981 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 1728, while the sum is 29.

The square root of 20621981 is about 4541.1431380215. The cubic root of 20621981 is about 274.2269462568.

The spelling of 20621981 in words is "twenty million, six hundred twenty-one thousand, nine hundred eighty-one".