21665247 has 4 divisors (see below), whose sum is σ = 28887000. Its totient is φ = 14443496.

The previous prime is 21665243. The next prime is 21665251. The reversal of 21665247 is 74256612.

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

It is an interprime number because it is at equal distance from previous prime (21665243) and next prime (21665251).

It is a cyclic number.

It is not a de Polignac number, because 21665247 - 2² = 21665243 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21665243) 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, 3610872 + ... + 3610877.

It is an arithmetic number, because the mean of its divisors is an integer number (7221750).

Almost surely, 2^21665247 is an apocalyptic number.

21665247 is a deficient number, since it is larger than the sum of its proper divisors (7221753).

21665247 is an equidigital number, since it uses as much as digits as its factorization.

21665247 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7221752.

The product of its digits is 20160, while the sum is 33.

The square root of 21665247 is about 4654.5941820958. The cubic root of 21665247 is about 278.7754645709.

It can be divided in two parts, 21665 and 247, that added together give a palindrome (21912).

The spelling of 21665247 in words is "twenty-one million, six hundred sixty-five thousand, two hundred forty-seven".