21688397 has 2 divisors, whose sum is σ = 21688398. Its totient is φ = 21688396.

The previous prime is 21688393. The next prime is 21688409. The reversal of 21688397 is 79388612.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 18498601 + 3189796 = 4301^2 + 1786^2 .

It is a cyclic number.

It is not a de Polignac number, because 21688397 - 2² = 21688393 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 (21688391) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10844198 + 10844199.

It is an arithmetic number, because the mean of its divisors is an integer number (10844199).

Almost surely, 2^21688397 is an apocalyptic number.

It is an amenable number.

21688397 is a deficient number, since it is larger than the sum of its proper divisors (1).

21688397 is an equidigital number, since it uses as much as digits as its factorization.

21688397 is an evil number, because the sum of its binary digits is even.

The product of its digits is 145152, while the sum is 44.

The square root of 21688397 is about 4657.0803085195. The cubic root of 21688397 is about 278.8747226858.

The spelling of 21688397 in words is "twenty-one million, six hundred eighty-eight thousand, three hundred ninety-seven".