21665617 has 4 divisors (see below), whose sum is σ = 21713076. Its totient is φ = 21618160.

The previous prime is 21665587. The next prime is 21665641. The reversal of 21665617 is 71656612.

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 8532241 + 13133376 = 2921^2 + 3624^2 .

It is a cyclic number.

It is not a de Polignac number, because 21665617 - 2⁷ = 21665489 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21665647) 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, 23038 + ... + 23959.

It is an arithmetic number, because the mean of its divisors is an integer number (5428269).

Almost surely, 2^21665617 is an apocalyptic number.

It is an amenable number.

21665617 is a deficient number, since it is larger than the sum of its proper divisors (47459).

21665617 is an equidigital number, since it uses as much as digits as its factorization.

21665617 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 47458.

The product of its digits is 15120, while the sum is 34.

The square root of 21665617 is about 4654.6339276038. The cubic root of 21665617 is about 278.7770515416. Note that the first 3 decimals are identical.

The spelling of 21665617 in words is "twenty-one million, six hundred sixty-five thousand, six hundred seventeen".