21213981 has 40 divisors (see below), whose sum is σ = 33802560. Its totient is φ = 13229568.

The previous prime is 21213971. The next prime is 21213991. The reversal of 21213981 is 18931212.

It is an interprime number because it is at equal distance from previous prime (21213971) and next prime (21213991).

It is not a de Polignac number, because 21213981 - 2⁷ = 21213853 is a prime.

It is a super-2 number, since 2×21213981² = 900065979736722, which contains 22 as substring.

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

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21213901) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 39 ways as a sum of consecutive naturals, for example, 109821 + ... + 110013.

It is an arithmetic number, because the mean of its divisors is an integer number (845064).

Almost surely, 2^21213981 is an apocalyptic number.

It is an amenable number.

21213981 is a deficient number, since it is larger than the sum of its proper divisors (12588579).

21213981 is a wasteful number, since it uses less digits than its factorization.

21213981 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 287 (or 278 counting only the distinct ones).

The product of its digits is 864, while the sum is 27.

The square root of 21213981 is about 4605.8637626400. The cubic root of 21213981 is about 276.8263279013.

The spelling of 21213981 in words is "twenty-one million, two hundred thirteen thousand, nine hundred eighty-one".