21509431 has 4 divisors (see below), whose sum is σ = 21751200. Its totient is φ = 21267664.

The previous prime is 21509419. The next prime is 21509447. The reversal of 21509431 is 13490512.

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

It is a cyclic number.

It is not a de Polignac number, because 21509431 - 2⁷ = 21509303 is a prime.

It is a super-2 number, since 2×21509431² = 925311243887522, 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 = 21509396 and 21509405.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21509731) 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, 120751 + ... + 120928.

It is an arithmetic number, because the mean of its divisors is an integer number (5437800).

Almost surely, 2^21509431 is an apocalyptic number.

21509431 is a deficient number, since it is larger than the sum of its proper divisors (241769).

21509431 is an equidigital number, since it uses as much as digits as its factorization.

21509431 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 241768.

The product of its (nonzero) digits is 1080, while the sum is 25.

The square root of 21509431 is about 4637.8261071325. The cubic root of 21509431 is about 278.1055402115.

Adding to 21509431 its reverse (13490512), we get a palindrome (34999943).

The spelling of 21509431 in words is "twenty-one million, five hundred nine thousand, four hundred thirty-one".