21457981 has 2 divisors, whose sum is σ = 21457982. Its totient is φ = 21457980.

The previous prime is 21457967. The next prime is 21457993. The reversal of 21457981 is 18975412.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 10791225 + 10666756 = 3285^2 + 3266^2 .

It is a cyclic number.

It is not a de Polignac number, because 21457981 - 2⁷ = 21457853 is a prime.

It is a super-2 number, since 2×21457981² = 920889897192722, which contains 22 as substring.

It is a self number, because there is not a number n which added to its sum of digits gives 21457981.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21457481) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10728990 + 10728991.

It is an arithmetic number, because the mean of its divisors is an integer number (10728991).

Almost surely, 2^21457981 is an apocalyptic number.

It is an amenable number.

21457981 is a deficient number, since it is larger than the sum of its proper divisors (1).

21457981 is an equidigital number, since it uses as much as digits as its factorization.

21457981 is an evil number, because the sum of its binary digits is even.

The product of its digits is 20160, while the sum is 37.

The square root of 21457981 is about 4632.2760064573. The cubic root of 21457981 is about 277.8836227687.

The spelling of 21457981 in words is "twenty-one million, four hundred fifty-seven thousand, nine hundred eighty-one".