21531431 has 4 divisors (see below), whose sum is σ = 21545664. Its totient is φ = 21517200.

The previous prime is 21531379. The next prime is 21531439. The reversal of 21531431 is 13413512.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21531431 is a prime.

It is a super-2 number, since 2×21531431² = 927205041815522, 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 (21531439) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 4535 + ... + 7976.

It is an arithmetic number, because the mean of its divisors is an integer number (5386416).

Almost surely, 2^21531431 is an apocalyptic number.

21531431 is a deficient number, since it is larger than the sum of its proper divisors (14233).

21531431 is a wasteful number, since it uses less digits than its factorization.

21531431 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 14232.

The product of its digits is 360, while the sum is 20.

The square root of 21531431 is about 4640.1973018397. The cubic root of 21531431 is about 278.2003240161.

Adding to 21531431 its reverse (13413512), we get a palindrome (34944943).

The spelling of 21531431 in words is "twenty-one million, five hundred thirty-one thousand, four hundred thirty-one".