21681385 has 12 divisors (see below), whose sum is σ = 28598724. Its totient is φ = 15767840.

The previous prime is 21681377. The next prime is 21681389. The reversal of 21681385 is 58318612.

It can be written as a sum of positive squares in 2 ways, for example, as 7027801 + 14653584 = 2651^2 + 3828^2 .

It is not a de Polignac number, because 21681385 - 2³ = 21681377 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21681385.

It is not an unprimeable number, because it can be changed into a prime (21681389) 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 in 11 ways as a sum of consecutive naturals, for example, 17314 + ... + 18523.

It is an arithmetic number, because the mean of its divisors is an integer number (2383227).

Almost surely, 2^21681385 is an apocalyptic number.

It is an amenable number.

21681385 is a deficient number, since it is larger than the sum of its proper divisors (6917339).

21681385 is a wasteful number, since it uses less digits than its factorization.

21681385 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 35864 (or 35853 counting only the distinct ones).

The product of its digits is 11520, while the sum is 34.

The square root of 21681385 is about 4656.3274154638. The cubic root of 21681385 is about 278.8446654416.

Adding to 21681385 its reverse (58318612), we get a palindrome (79999997).

It can be divided in two parts, 2168 and 1385, that added together give a palindrome (3553).

The spelling of 21681385 in words is "twenty-one million, six hundred eighty-one thousand, three hundred eighty-five".