21657217 has 4 divisors (see below), whose sum is σ = 21752020. Its totient is φ = 21562416.

The previous prime is 21657211. The next prime is 21657221. The reversal of 21657217 is 71275612.

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

It can be written as a sum of positive squares in 2 ways, for example, as 20875761 + 781456 = 4569^2 + 884^2 .

It is a cyclic number.

It is not a de Polignac number, because 21657217 - 2²³ = 13268609 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21657211) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 47058 + ... + 47515.

It is an arithmetic number, because the mean of its divisors is an integer number (5438005).

Almost surely, 2^21657217 is an apocalyptic number.

It is an amenable number.

21657217 is a deficient number, since it is larger than the sum of its proper divisors (94803).

21657217 is an equidigital number, since it uses as much as digits as its factorization.

21657217 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 94802.

The product of its digits is 5880, while the sum is 31.

The square root of 21657217 is about 4653.7315135276. The cubic root of 21657217 is about 278.7410185661.

The spelling of 21657217 in words is "twenty-one million, six hundred fifty-seven thousand, two hundred seventeen".