21428681 has 2 divisors, whose sum is σ = 21428682. Its totient is φ = 21428680.

The previous prime is 21428621. The next prime is 21428683. The reversal of 21428681 is 18682412.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 12187081 + 9241600 = 3491^2 + 3040^2 .

It is a cyclic number.

It is not a de Polignac number, because 21428681 - 2¹⁰ = 21427657 is a prime.

It is a super-2 number, since 2×21428681² = 918376738799522, which contains 22 as substring.

Together with 21428683, it forms a pair of twin primes.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (21428683) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10714340 + 10714341.

It is an arithmetic number, because the mean of its divisors is an integer number (10714341).

Almost surely, 2^21428681 is an apocalyptic number.

It is an amenable number.

21428681 is a deficient number, since it is larger than the sum of its proper divisors (1).

21428681 is an equidigital number, since it uses as much as digits as its factorization.

21428681 is an evil number, because the sum of its binary digits is even.

The product of its digits is 6144, while the sum is 32.

The square root of 21428681 is about 4629.1123339146. The cubic root of 21428681 is about 277.7570855638.

The spelling of 21428681 in words is "twenty-one million, four hundred twenty-eight thousand, six hundred eighty-one".