21015865 has 16 divisors (see below), whose sum is σ = 28098000. Its totient is φ = 14982912.

The previous prime is 21015857. The next prime is 21015871. The reversal of 21015865 is 56851012.

It can be written as a sum of positive squares in 8 ways, for example, as 2534464 + 18481401 = 1592^2 + 4299^2 .

It is a cyclic number.

It is not a de Polignac number, because 21015865 - 2³ = 21015857 is a prime.

It is an unprimeable number.

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, 3690 + ... + 7459.

It is an arithmetic number, because the mean of its divisors is an integer number (1756125).

Almost surely, 2^21015865 is an apocalyptic number.

It is an amenable number.

21015865 is a deficient number, since it is larger than the sum of its proper divisors (7082135).

21015865 is a wasteful number, since it uses less digits than its factorization.

21015865 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 11196.

The product of its (nonzero) digits is 2400, while the sum is 28.

The square root of 21015865 is about 4584.3063815587. The cubic root of 21015865 is about 275.9618768670.

Adding to 21015865 its reverse (56851012), we get a palindrome (77866877).

The spelling of 21015865 in words is "twenty-one million, fifteen thousand, eight hundred sixty-five".