21005641 has 4 divisors (see below), whose sum is σ = 21222292. Its totient is φ = 20788992.

The previous prime is 21005627. The next prime is 21005651. The reversal of 21005641 is 14650012.

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 10214416 + 10791225 = 3196^2 + 3285^2 .

It is a cyclic number.

It is not a de Polignac number, because 21005641 - 2⁵ = 21005609 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21005651) 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, 108180 + ... + 108373.

It is an arithmetic number, because the mean of its divisors is an integer number (5305573).

Almost surely, 2^21005641 is an apocalyptic number.

It is an amenable number.

21005641 is a deficient number, since it is larger than the sum of its proper divisors (216651).

21005641 is an equidigital number, since it uses as much as digits as its factorization.

21005641 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 216650.

The product of its (nonzero) digits is 240, while the sum is 19.

The square root of 21005641 is about 4583.1911371881. The cubic root of 21005641 is about 275.9171187460.

Adding to 21005641 its reverse (14650012), we get a palindrome (35655653).

The spelling of 21005641 in words is "twenty-one million, five thousand, six hundred forty-one".