21921665 has 4 divisors (see below), whose sum is σ = 26306004. Its totient is φ = 17537328.

The previous prime is 21921659. The next prime is 21921667. The reversal of 21921665 is 56612912.

It is a happy number.

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 5013121 + 16908544 = 2239^2 + 4112^2 .

It is a cyclic number.

It is not a de Polignac number, because 21921665 - 2¹² = 21917569 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21921667) 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, 2192162 + ... + 2192171.

It is an arithmetic number, because the mean of its divisors is an integer number (6576501).

Almost surely, 2^21921665 is an apocalyptic number.

It is an amenable number.

21921665 is a deficient number, since it is larger than the sum of its proper divisors (4384339).

21921665 is an equidigital number, since it uses as much as digits as its factorization.

21921665 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4384338.

The product of its digits is 6480, while the sum is 32.

The square root of 21921665 is about 4682.0577740989. The cubic root of 21921665 is about 279.8709652048.

The spelling of 21921665 in words is "twenty-one million, nine hundred twenty-one thousand, six hundred sixty-five".