20509981 has 2 divisors, whose sum is σ = 20509982. Its totient is φ = 20509980.

The previous prime is 20509967. The next prime is 20510027. The reversal of 20509981 is 18990502.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 18232900 + 2277081 = 4270^2 + 1509^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-20509981 is a prime.

It is a super-2 number, since 2×20509981² = 841318641240722, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (20509921) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10254990 + 10254991.

It is an arithmetic number, because the mean of its divisors is an integer number (10254991).

Almost surely, 2^20509981 is an apocalyptic number.

It is an amenable number.

20509981 is a deficient number, since it is larger than the sum of its proper divisors (1).

20509981 is an equidigital number, since it uses as much as digits as its factorization.

20509981 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 6480, while the sum is 34.

The square root of 20509981 is about 4528.7946520018. The cubic root of 20509981 is about 273.7295936360.

The spelling of 20509981 in words is "twenty million, five hundred nine thousand, nine hundred eighty-one".