21989681 has 6 divisors (see below), whose sum is σ = 25579890. Its totient is φ = 18848256.

The previous prime is 21989659. The next prime is 21989689. The reversal of 21989681 is 18698912.

It can be written as a sum of positive squares in only one way, i.e., 11621281 + 10368400 = 3409^2 + 3220^2 .

It is not a de Polignac number, because 21989681 - 2¹⁰ = 21988657 is a prime.

It is a super-2 number, since 2×21989681² = 967092140963522, which contains 22 as substring.

It is not an unprimeable number, because it can be changed into a prime (21989689) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 224336 + ... + 224433.

It is an arithmetic number, because the mean of its divisors is an integer number (4263315).

Almost surely, 2^21989681 is an apocalyptic number.

It is an amenable number.

21989681 is a deficient number, since it is larger than the sum of its proper divisors (3590209).

21989681 is an equidigital number, since it uses as much as digits as its factorization.

21989681 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 448783 (or 448776 counting only the distinct ones).

The product of its digits is 62208, while the sum is 44.

The square root of 21989681 is about 4689.3156217086. The cubic root of 21989681 is about 280.1601167538.

The spelling of 21989681 in words is "twenty-one million, nine hundred eighty-nine thousand, six hundred eighty-one".