21681389 has 2 divisors, whose sum is σ = 21681390. Its totient is φ = 21681388.

The previous prime is 21681377. The next prime is 21681421. The reversal of 21681389 is 98318612.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 21548164 + 133225 = 4642^2 + 365^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21681389 is a prime.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21681689) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10840694 + 10840695.

It is an arithmetic number, because the mean of its divisors is an integer number (10840695).

Almost surely, 2^21681389 is an apocalyptic number.

It is an amenable number.

21681389 is a deficient number, since it is larger than the sum of its proper divisors (1).

21681389 is an equidigital number, since it uses as much as digits as its factorization.

21681389 is an evil number, because the sum of its binary digits is even.

The product of its digits is 20736, while the sum is 38.

The square root of 21681389 is about 4656.3278449869. The cubic root of 21681389 is about 278.8446825896.

The spelling of 21681389 in words is "twenty-one million, six hundred eighty-one thousand, three hundred eighty-nine".