21011981 has 2 divisors, whose sum is σ = 21011982. Its totient is φ = 21011980.

The previous prime is 21011959. The next prime is 21012053. The reversal of 21011981 is 18911012.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 17556100 + 3455881 = 4190^2 + 1859^2 .

It is a cyclic number.

It is not a de Polignac number, because 21011981 - 2⁶ = 21011917 is a prime.

It is a super-2 number, since 2×21011981² = 883006691088722, which contains 22 as substring.

It is a Chen prime.

It is equal to p_1330682 and since 21011981 and 1330682 have the same sum of digits, it is a Honaker prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21011951) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10505990 + 10505991.

It is an arithmetic number, because the mean of its divisors is an integer number (10505991).

Almost surely, 2^21011981 is an apocalyptic number.

It is an amenable number.

21011981 is a deficient number, since it is larger than the sum of its proper divisors (1).

21011981 is an equidigital number, since it uses as much as digits as its factorization.

21011981 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 144, while the sum is 23.

The square root of 21011981 is about 4583.8827428284. The cubic root of 21011981 is about 275.9448753943.

Adding to 21011981 its reverse (18911012), we get a palindrome (39922993).

The spelling of 21011981 in words is "twenty-one million, eleven thousand, nine hundred eighty-one".