21155981 has 16 divisors (see below), whose sum is σ = 27230208. Its totient is φ = 15951600.

The previous prime is 21155977. The next prime is 21155999. The reversal of 21155981 is 18955112.

It is not a de Polignac number, because 21155981 - 2² = 21155977 is a prime.

It is a super-2 number, since 2×21155981² = 895151064144722, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21155921) 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, 2045 + ... + 6818.

It is an arithmetic number, because the mean of its divisors is an integer number (1701888).

Almost surely, 2^21155981 is an apocalyptic number.

It is an amenable number.

21155981 is a deficient number, since it is larger than the sum of its proper divisors (6074227).

21155981 is a wasteful number, since it uses less digits than its factorization.

21155981 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 8912.

The product of its digits is 3600, while the sum is 32.

The square root of 21155981 is about 4599.5631314289. The cubic root of 21155981 is about 276.5738123263.

The spelling of 21155981 in words is "twenty-one million, one hundred fifty-five thousand, nine hundred eighty-one".