21291981 has 8 divisors (see below), whose sum is σ = 28452480. Its totient is φ = 14163072.

The previous prime is 21291971. The next prime is 21292013. The reversal of 21291981 is 18919212.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 21291981 - 2¹⁸ = 21029837 is a prime.

It is a super-2 number, since 2×21291981² = 906696909808722, which contains 22 as substring.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21291961) 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 7 ways as a sum of consecutive naturals, for example, 6276 + ... + 9053.

It is an arithmetic number, because the mean of its divisors is an integer number (3556560).

Almost surely, 2^21291981 is an apocalyptic number.

It is an amenable number.

21291981 is a deficient number, since it is larger than the sum of its proper divisors (7160499).

21291981 is a wasteful number, since it uses less digits than its factorization.

21291981 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 15795.

The product of its digits is 2592, while the sum is 33.

The square root of 21291981 is about 4614.3234607036. The cubic root of 21291981 is about 277.1651931626.

The spelling of 21291981 in words is "twenty-one million, two hundred ninety-one thousand, nine hundred eighty-one".