21109831 has 4 divisors (see below), whose sum is σ = 21123432. Its totient is φ = 21096232.

The previous prime is 21109799. The next prime is 21109841. The reversal of 21109831 is 13890112.

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 21109831 - 2⁵ = 21109799 is a prime.

It is a super-2 number, since 2×21109831² = 891249929697122, 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 = 21109796 and 21109805.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21109841) 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, 4120 + ... + 7693.

It is an arithmetic number, because the mean of its divisors is an integer number (5280858).

Almost surely, 2^21109831 is an apocalyptic number.

21109831 is a deficient number, since it is larger than the sum of its proper divisors (13601).

21109831 is a wasteful number, since it uses less digits than its factorization.

21109831 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 13600.

The product of its (nonzero) digits is 432, while the sum is 25.

The square root of 21109831 is about 4594.5436117203. The cubic root of 21109831 is about 276.3725583828.

Adding to 21109831 its reverse (13890112), we get a palindrome (34999943).

The spelling of 21109831 in words is "twenty-one million, one hundred nine thousand, eight hundred thirty-one".