21431981 has 8 divisors (see below), whose sum is σ = 22986720. Its totient is φ = 19919952.

The previous prime is 21431959. The next prime is 21431983. The reversal of 21431981 is 18913412.

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

It is a cyclic number.

It is not a de Polignac number, because 21431981 - 2¹⁰ = 21430957 is a prime.

It is a super-2 number, since 2×21431981² = 918659619168722, which contains 22 as substring.

It is a Duffinian number.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21431983) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 9635 + ... + 11648.

It is an arithmetic number, because the mean of its divisors is an integer number (2873340).

Almost surely, 2^21431981 is an apocalyptic number.

It is an amenable number.

21431981 is a deficient number, since it is larger than the sum of its proper divisors (1554739).

21431981 is a wasteful number, since it uses less digits than its factorization.

21431981 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 21355.

The product of its digits is 1728, while the sum is 29.

The square root of 21431981 is about 4629.4687600199. The cubic root of 21431981 is about 277.7713429561.

Subtracting from 21431981 its reverse (18913412), we obtain a square (2518569 = 1587²).

The spelling of 21431981 in words is "twenty-one million, four hundred thirty-one thousand, nine hundred eighty-one".