20909981 has 2 divisors, whose sum is σ = 20909982. Its totient is φ = 20909980.

The previous prime is 20909969. The next prime is 20909989. The reversal of 20909981 is 18990902.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 20839225 + 70756 = 4565^2 + 266^2 .

It is a cyclic number.

It is not a de Polignac number, because 20909981 - 2¹⁰ = 20908957 is a prime.

It is a super-2 number, since 2×20909981² = 874454610840722, which contains 22 as substring.

It is a Sophie Germain prime.

It is a Chen prime.

It is a Curzon number.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (20909989) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10454990 + 10454991.

It is an arithmetic number, because the mean of its divisors is an integer number (10454991).

Almost surely, 2^20909981 is an apocalyptic number.

It is an amenable number.

20909981 is a deficient number, since it is larger than the sum of its proper divisors (1).

20909981 is an equidigital number, since it uses as much as digits as its factorization.

20909981 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 11664, while the sum is 38.

The square root of 20909981 is about 4572.7432685424. The cubic root of 20909981 is about 275.4976377785.

The spelling of 20909981 in words is "twenty million, nine hundred nine thousand, nine hundred eighty-one".