21161981 has 2 divisors, whose sum is σ = 21161982. Its totient is φ = 21161980.

The previous prime is 21161971. The next prime is 21161983. The reversal of 21161981 is 18916112.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 20025625 + 1136356 = 4475^2 + 1066^2 .

It is a cyclic number.

It is not a de Polignac number, because 21161981 - 2¹⁰ = 21160957 is a prime.

It is a super-2 number, since 2×21161981² = 895658879688722, which contains 22 as substring.

Together with 21161983, 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 (21161983) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10580990 + 10580991.

It is an arithmetic number, because the mean of its divisors is an integer number (10580991).

Almost surely, 2^21161981 is an apocalyptic number.

It is an amenable number.

21161981 is a deficient number, since it is larger than the sum of its proper divisors (1).

21161981 is an equidigital number, since it uses as much as digits as its factorization.

21161981 is an evil number, because the sum of its binary digits is even.

The product of its digits is 864, while the sum is 29.

The square root of 21161981 is about 4600.2153210475. The cubic root of 21161981 is about 276.5999560131.

The spelling of 21161981 in words is "twenty-one million, one hundred sixty-one thousand, nine hundred eighty-one".