21515639 has 2 divisors, whose sum is σ = 21515640. Its totient is φ = 21515638.

The previous prime is 21515633. The next prime is 21515647. The reversal of 21515639 is 93651512.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21515639 - 2¹⁶ = 21450103 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21515633) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10757819 + 10757820.

It is an arithmetic number, because the mean of its divisors is an integer number (10757820).

Almost surely, 2^21515639 is an apocalyptic number.

21515639 is a deficient number, since it is larger than the sum of its proper divisors (1).

21515639 is an equidigital number, since it uses as much as digits as its factorization.

21515639 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 8100, while the sum is 32.

The square root of 21515639 is about 4638.4953379302. The cubic root of 21515639 is about 278.1322930209.

The spelling of 21515639 in words is "twenty-one million, five hundred fifteen thousand, six hundred thirty-nine".