21005723 has 4 divisors (see below), whose sum is σ = 21258888. Its totient is φ = 20752560.

The previous prime is 21005713. The next prime is 21005737. The reversal of 21005723 is 32750012.

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

It is a cyclic number.

It is not a de Polignac number, because 21005723 - 2⁴ = 21005707 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21005695 and 21005704.

It is not an unprimeable number, because it can be changed into a prime (21005713) 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, 126458 + ... + 126623.

It is an arithmetic number, because the mean of its divisors is an integer number (5314722).

Almost surely, 2^21005723 is an apocalyptic number.

21005723 is a deficient number, since it is larger than the sum of its proper divisors (253165).

21005723 is an equidigital number, since it uses as much as digits as its factorization.

21005723 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 253164.

The product of its (nonzero) digits is 420, while the sum is 20.

The square root of 21005723 is about 4583.2000829115. The cubic root of 21005723 is about 275.9174777793.

Adding to 21005723 its reverse (32750012), we get a palindrome (53755735).

The spelling of 21005723 in words is "twenty-one million, five thousand, seven hundred twenty-three".