21640247 has 4 divisors (see below), whose sum is σ = 21649896. Its totient is φ = 21630600.

The previous prime is 21640237. The next prime is 21640249. The reversal of 21640247 is 74204612.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It is a cyclic number.

It is not a de Polignac number, because 21640247 - 2⁴ = 21640231 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21640249) 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, 497 + ... + 6597.

It is an arithmetic number, because the mean of its divisors is an integer number (5412474).

Almost surely, 2^21640247 is an apocalyptic number.

21640247 is a deficient number, since it is larger than the sum of its proper divisors (9649).

21640247 is an equidigital number, since it uses as much as digits as its factorization.

21640247 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 9648.

The product of its (nonzero) digits is 2688, while the sum is 26.

The square root of 21640247 is about 4651.9078881680. The cubic root of 21640247 is about 278.6681949416.

Adding to 21640247 its reverse (74204612), we get a palindrome (95844859).

The spelling of 21640247 in words is "twenty-one million, six hundred forty thousand, two hundred forty-seven".