21621881 has 2 divisors, whose sum is σ = 21621882. Its totient is φ = 21621880.

The previous prime is 21621871. The next prime is 21621923. The reversal of 21621881 is 18812612.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 17977600 + 3644281 = 4240^2 + 1909^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21621881 is a prime.

It is a super-2 number, since 2×21621881² = 935011475956322, which contains 22 as substring.

It is not a weakly prime, because it can be changed into another prime (21621851) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10810940 + 10810941.

It is an arithmetic number, because the mean of its divisors is an integer number (10810941).

Almost surely, 2^21621881 is an apocalyptic number.

It is an amenable number.

21621881 is a deficient number, since it is larger than the sum of its proper divisors (1).

21621881 is an equidigital number, since it uses as much as digits as its factorization.

21621881 is an evil number, because the sum of its binary digits is even.

The product of its digits is 1536, while the sum is 29.

The square root of 21621881 is about 4649.9334403839. The cubic root of 21621881 is about 278.5893377297.

The spelling of 21621881 in words is "twenty-one million, six hundred twenty-one thousand, eight hundred eighty-one".