21681387 has 48 divisors (see below), whose sum is σ = 40255488. Its totient is φ = 10929600.

The previous prime is 21681377. The next prime is 21681389. The reversal of 21681387 is 78318612.

It is not a de Polignac number, because 21681387 - 2¹⁶ = 21615851 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21681389) by changing a digit.

It is a polite number, since it can be written in 47 ways as a sum of consecutive naturals, for example, 18262 + ... + 19412.

It is an arithmetic number, because the mean of its divisors is an integer number (838656).

Almost surely, 2^21681387 is an apocalyptic number.

21681387 is a deficient number, since it is larger than the sum of its proper divisors (18574101).

21681387 is a wasteful number, since it uses less digits than its factorization.

21681387 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1200 (or 1197 counting only the distinct ones).

The product of its digits is 16128, while the sum is 36.

The square root of 21681387 is about 4656.3276302253. The cubic root of 21681387 is about 278.8446740156.

Adding to 21681387 its reverse (78318612), we get a palindrome (99999999).

The spelling of 21681387 in words is "twenty-one million, six hundred eighty-one thousand, three hundred eighty-seven".