21688391 has 2 divisors, whose sum is σ = 21688392. Its totient is φ = 21688390.

The previous prime is 21688343. The next prime is 21688393. The reversal of 21688391 is 19388612.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21688391 - 2¹⁸ = 21426247 is a prime.

It is a Sophie Germain prime.

Together with 21688393, it forms a pair of twin primes.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21688393) by changing a digit.

It is a good prime.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10844195 + 10844196.

It is an arithmetic number, because the mean of its divisors is an integer number (10844196).

Almost surely, 2^21688391 is an apocalyptic number.

21688391 is a deficient number, since it is larger than the sum of its proper divisors (1).

21688391 is an equidigital number, since it uses as much as digits as its factorization.

21688391 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 21688391 is about 4657.0796643390. The cubic root of 21688391 is about 278.8746969693.

The spelling of 21688391 in words is "twenty-one million, six hundred eighty-eight thousand, three hundred ninety-one".