21506639 has 12 divisors (see below), whose sum is σ = 27292968. Its totient is φ = 16758000.

The previous prime is 21506629. The next prime is 21506647. The reversal of 21506639 is 93660512.

It is a happy number.

It is not a de Polignac number, because 21506639 - 2¹⁶ = 21441103 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 (21506609) 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 11 ways as a sum of consecutive naturals, for example, 19412 + ... + 20489.

It is an arithmetic number, because the mean of its divisors is an integer number (2274414).

Almost surely, 2^21506639 is an apocalyptic number.

21506639 is a deficient number, since it is larger than the sum of its proper divisors (5786329).

21506639 is a wasteful number, since it uses less digits than its factorization.

21506639 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 39926 (or 39919 counting only the distinct ones).

The product of its (nonzero) digits is 9720, while the sum is 32.

The square root of 21506639 is about 4637.5250942717. The cubic root of 21506639 is about 278.0935066642.

The spelling of 21506639 in words is "twenty-one million, five hundred six thousand, six hundred thirty-nine".