21645881 has 2 divisors, whose sum is σ = 21645882. Its totient is φ = 21645880.

The previous prime is 21645859. The next prime is 21645901. The reversal of 21645881 is 18854612.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21529600 + 116281 = 4640^2 + 341^2 .

It is a cyclic number.

It is not a de Polignac number, because 21645881 - 2⁶ = 21645817 is a prime.

It is a super-2 number, since 2×21645881² = 937088328532322, which contains 22 as substring.

It is not a weakly prime, because it can be changed into another prime (21645181) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10822940 + 10822941.

It is an arithmetic number, because the mean of its divisors is an integer number (10822941).

Almost surely, 2^21645881 is an apocalyptic number.

It is an amenable number.

21645881 is a deficient number, since it is larger than the sum of its proper divisors (1).

21645881 is an equidigital number, since it uses as much as digits as its factorization.

21645881 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 15360, while the sum is 35.

The square root of 21645881 is about 4652.5134067512. The cubic root of 21645881 is about 278.6923764336.

The spelling of 21645881 in words is "twenty-one million, six hundred forty-five thousand, eight hundred eighty-one".