21381205 has 4 divisors (see below), whose sum is σ = 25657452. Its totient is φ = 17104960.

The previous prime is 21381203. The next prime is 21381209. The reversal of 21381205 is 50218312.

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 6589489 + 14791716 = 2567^2 + 3846^2 .

It is not a de Polignac number, because 21381205 - 2¹ = 21381203 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21381203) 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, 2138116 + ... + 2138125.

It is an arithmetic number, because the mean of its divisors is an integer number (6414363).

Almost surely, 2^21381205 is an apocalyptic number.

It is an amenable number.

21381205 is a deficient number, since it is larger than the sum of its proper divisors (4276247).

21381205 is an equidigital number, since it uses as much as digits as its factorization.

21381205 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4276246.

The product of its (nonzero) digits is 480, while the sum is 22.

The square root of 21381205 is about 4623.9815094786. The cubic root of 21381205 is about 277.5518070090.

Adding to 21381205 its reverse (50218312), we get a palindrome (71599517).

It can be divided in two parts, 213 and 81205, that added together give a palindrome (81418).

The spelling of 21381205 in words is "twenty-one million, three hundred eighty-one thousand, two hundred five".