21255691 has 4 divisors (see below), whose sum is σ = 21351232. Its totient is φ = 21160152.

The previous prime is 21255679. The next prime is 21255721. The reversal of 21255691 is 19655212.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 21255691 - 2¹⁵ = 21222923 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21255791) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 47436 + ... + 47881.

It is an arithmetic number, because the mean of its divisors is an integer number (5337808).

Almost surely, 2^21255691 is an apocalyptic number.

21255691 is a deficient number, since it is larger than the sum of its proper divisors (95541).

21255691 is an equidigital number, since it uses as much as digits as its factorization.

21255691 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 95540.

The product of its digits is 5400, while the sum is 31.

The square root of 21255691 is about 4610.3894629413. The cubic root of 21255691 is about 277.0076370583.

The spelling of 21255691 in words is "twenty-one million, two hundred fifty-five thousand, six hundred ninety-one".