21538881 has 81 divisors (see below), whose sum is σ = 41630121. Its totient is φ = 10692864.

The previous prime is 21538873. The next prime is 21538883. The reversal of 21538881 is 18883512.

The square root of 21538881 is 4641.

It is a perfect power (a square), and thus also a powerful number.

21538881 is nontrivially palindromic in base 16.

It can be written as a sum of positive squares in 4 ways, for example, as 194481 + 21344400 = 441^2 + 4620^2 .

It is not a de Polignac number, because 21538881 - 2³ = 21538873 is a prime.

It is a super-2 number, since 2×21538881² = 927846789464322, which contains 22 as substring.

Its product of digits (15360) is a multiple of the sum of its prime divisors (40).

It is not an unprimeable number, because it can be changed into a prime (21538883) by changing a digit.

It is a polite number, since it can be written in 80 ways as a sum of consecutive naturals, for example, 1266985 + ... + 1267001.

Almost surely, 2^21538881 is an apocalyptic number.

21538881 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

21538881 is the 4641-st square number.

21538881 is the 2321-st centered octagonal number.

It is an amenable number.

21538881 is a deficient number, since it is larger than the sum of its proper divisors (20091240).

21538881 is a wasteful number, since it uses less digits than its factorization.

21538881 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 80 (or 40 counting only the distinct ones).

The product of its digits is 15360, while the sum is 36.

The cubic root of 21538881 is about 278.2324066247.

Multiplying 21538881 by its sum of digits (36), we get a square (775399716 = 27846²).

The spelling of 21538881 in words is "twenty-one million, five hundred thirty-eight thousand, eight hundred eighty-one".