21205639 has 16 divisors (see below), whose sum is σ = 26252800. Its totient is φ = 16679520.

The previous prime is 21205637. The next prime is 21205649. The reversal of 21205639 is 93650212.

It is not a de Polignac number, because 21205639 - 2¹ = 21205637 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21205633) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 17524 + ... + 18694.

It is an arithmetic number, because the mean of its divisors is an integer number (1640800).

Almost surely, 2^21205639 is an apocalyptic number.

21205639 is a deficient number, since it is larger than the sum of its proper divisors (5047161).

21205639 is a wasteful number, since it uses less digits than its factorization.

21205639 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1390.

The product of its (nonzero) digits is 3240, while the sum is 28.

The square root of 21205639 is about 4604.9580888429. The cubic root of 21205639 is about 276.7900375576.

The spelling of 21205639 in words is "twenty-one million, two hundred five thousand, six hundred thirty-nine".