21331531 has 4 divisors (see below), whose sum is σ = 22972432. Its totient is φ = 19690632.

The previous prime is 21331523. The next prime is 21331537. The reversal of 21331531 is 13513312.

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

It is not a de Polignac number, because 21331531 - 2³ = 21331523 is a prime.

It is a super-2 number, since 2×21331531² = 910068429607922, which contains 22 as substring.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21331499 and 21331508.

It is not an unprimeable number, because it can be changed into a prime (21331537) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 820431 + ... + 820456.

It is an arithmetic number, because the mean of its divisors is an integer number (5743108).

Almost surely, 2^21331531 is an apocalyptic number.

21331531 is a deficient number, since it is larger than the sum of its proper divisors (1640901).

21331531 is a wasteful number, since it uses less digits than its factorization.

21331531 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1640900.

The product of its digits is 270, while the sum is 19.

The square root of 21331531 is about 4618.6070410893. The cubic root of 21331531 is about 277.3366990944.

Adding to 21331531 its reverse (13513312), we get a palindrome (34844843).

It can be divided in two parts, 2133 and 1531, that multiplied together give a palindrome (3265623).

The spelling of 21331531 in words is "twenty-one million, three hundred thirty-one thousand, five hundred thirty-one".