21568639 has 2 divisors, whose sum is σ = 21568640. Its totient is φ = 21568638.

The previous prime is 21568633. The next prime is 21568643. The reversal of 21568639 is 93686512.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21568639 - 2²³ = 13180031 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21568633) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10784319 + 10784320.

It is an arithmetic number, because the mean of its divisors is an integer number (10784320).

Almost surely, 2^21568639 is an apocalyptic number.

21568639 is a deficient number, since it is larger than the sum of its proper divisors (1).

21568639 is an equidigital number, since it uses as much as digits as its factorization.

21568639 is an evil number, because the sum of its binary digits is even.

The product of its digits is 77760, while the sum is 40.

The square root of 21568639 is about 4644.2048835080. The cubic root of 21568639 is about 278.3604824494.

The spelling of 21568639 in words is "twenty-one million, five hundred sixty-eight thousand, six hundred thirty-nine".