21123205 has 12 divisors (see below), whose sum is σ = 25668900. Its totient is φ = 16687104.

The previous prime is 21123187. The next prime is 21123211. The reversal of 21123205 is 50232112.

It can be written as a sum of positive squares in 6 ways, for example, as 91809 + 21031396 = 303^2 + 4586^2 .

It is not a de Polignac number, because 21123205 - 2⁵ = 21123173 is a prime.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 46821 + ... + 47269.

It is an arithmetic number, because the mean of its divisors is an integer number (2139075).

Almost surely, 2^21123205 is an apocalyptic number.

It is an amenable number.

21123205 is a deficient number, since it is larger than the sum of its proper divisors (4545695).

21123205 is an frugal number, since it uses more digits than its factorization.

21123205 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 648 (or 551 counting only the distinct ones).

The product of its (nonzero) digits is 120, while the sum is 16.

The square root of 21123205 is about 4595.9988033071. The cubic root of 21123205 is about 276.4309107574.

Adding to 21123205 its reverse (50232112), we get a palindrome (71355317).

The spelling of 21123205 in words is "twenty-one million, one hundred twenty-three thousand, two hundred five".