21390643 has 4 divisors (see below), whose sum is σ = 21912408. Its totient is φ = 20868880.

The previous prime is 21390631. The next prime is 21390647. The reversal of 21390643 is 34609312.

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-21390643 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21390647) 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, 260821 + ... + 260902.

It is an arithmetic number, because the mean of its divisors is an integer number (5478102).

Almost surely, 2^21390643 is an apocalyptic number.

21390643 is a deficient number, since it is larger than the sum of its proper divisors (521765).

21390643 is an equidigital number, since it uses as much as digits as its factorization.

21390643 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 521764.

The product of its (nonzero) digits is 3888, while the sum is 28.

The square root of 21390643 is about 4625.0019459455. The cubic root of 21390643 is about 277.5926395784.

Adding to 21390643 its reverse (34609312), we get a palindrome (55999955).

The spelling of 21390643 in words is "twenty-one million, three hundred ninety thousand, six hundred forty-three".