21018643 has 4 divisors (see below), whose sum is σ = 21046608. Its totient is φ = 20990680.

The previous prime is 21018631. The next prime is 21018659. The reversal of 21018643 is 34681012.

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

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21018643 is a prime.

It is a super-2 number, since 2×21018643² = 883566707122898, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21018623) 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, 12823 + ... + 14368.

It is an arithmetic number, because the mean of its divisors is an integer number (5261652).

Almost surely, 2^21018643 is an apocalyptic number.

21018643 is a deficient number, since it is larger than the sum of its proper divisors (27965).

21018643 is an equidigital number, since it uses as much as digits as its factorization.

21018643 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 27964.

The product of its (nonzero) digits is 1152, while the sum is 25.

The square root of 21018643 is about 4584.6093617668. The cubic root of 21018643 is about 275.9740357498.

Adding to 21018643 its reverse (34681012), we get a palindrome (55699655).

The spelling of 21018643 in words is "twenty-one million, eighteen thousand, six hundred forty-three".