21555981 has 12 divisors (see below), whose sum is σ = 31362240. Its totient is φ = 14266440.

The previous prime is 21555967. The next prime is 21556009. The reversal of 21555981 is 18955512.

It is a happy number.

21555981 is a `hidden beast` number, since 21 + 5 + 559 + 81 = 666.

It is not a de Polignac number, because 21555981 - 2⁶ = 21555917 is a prime.

It is a super-2 number, since 2×21555981² = 929320633744722, which contains 22 as substring.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21555931) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 7365 + ... + 9866.

It is an arithmetic number, because the mean of its divisors is an integer number (2613520).

Almost surely, 2^21555981 is an apocalyptic number.

It is an amenable number.

21555981 is a deficient number, since it is larger than the sum of its proper divisors (9806259).

21555981 is a wasteful number, since it uses less digits than its factorization.

21555981 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 17376 (or 17373 counting only the distinct ones).

The product of its digits is 18000, while the sum is 36.

The square root of 21555981 is about 4642.8419098651. The cubic root of 21555981 is about 278.3060179328.

The spelling of 21555981 in words is "twenty-one million, five hundred fifty-five thousand, nine hundred eighty-one".