21516381 has 16 divisors (see below), whose sum is σ = 31960000. Its totient is φ = 14306544.

The previous prime is 21516359. The next prime is 21516403. The reversal of 21516381 is 18361512.

21516381 is a `hidden beast` number, since 21 + 5 + 1 + 638 + 1 = 666.

It is an interprime number because it is at equal distance from previous prime (21516359) and next prime (21516403).

It is not a de Polignac number, because 21516381 - 2¹⁰ = 21515357 is a prime.

It is a super-2 number, since 2×21516381² = 925909302674322, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21516301) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 12675 + ... + 14271.

It is an arithmetic number, because the mean of its divisors is an integer number (1997500).

Almost surely, 2^21516381 is an apocalyptic number.

It is an amenable number.

21516381 is a deficient number, since it is larger than the sum of its proper divisors (10443619).

21516381 is a wasteful number, since it uses less digits than its factorization.

21516381 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2105 (or 2099 counting only the distinct ones).

The product of its digits is 1440, while the sum is 27.

The square root of 21516381 is about 4638.5753200740. The cubic root of 21516381 is about 278.1354902579.

Adding to 21516381 its reverse (18361512), we get a palindrome (39877893).

The spelling of 21516381 in words is "twenty-one million, five hundred sixteen thousand, three hundred eighty-one".