20531981 has 2 divisors, whose sum is σ = 20531982. Its totient is φ = 20531980.

The previous prime is 20531947. The next prime is 20531983. The reversal of 20531981 is 18913502.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 16883881 + 3648100 = 4109^2 + 1910^2 .

It is a cyclic number.

It is not a de Polignac number, because 20531981 - 2²² = 16337677 is a prime.

It is a super-2 number, since 2×20531981² = 843124487568722, which contains 22 as substring.

Together with 20531983, it forms a pair of twin primes.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (20531983) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10265990 + 10265991.

It is an arithmetic number, because the mean of its divisors is an integer number (10265991).

Almost surely, 2^20531981 is an apocalyptic number.

It is an amenable number.

20531981 is a deficient number, since it is larger than the sum of its proper divisors (1).

20531981 is an equidigital number, since it uses as much as digits as its factorization.

20531981 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 2160, while the sum is 29.

The square root of 20531981 is about 4531.2229033673. Note that the first 3 decimals coincide. The cubic root of 20531981 is about 273.8274305406.

It can be divided in two parts, 20531 and 981, that added together give a palindrome (21512).

The spelling of 20531981 in words is "twenty million, five hundred thirty-one thousand, nine hundred eighty-one".