21691231 has 4 divisors (see below), whose sum is σ = 22634352. Its totient is φ = 20748112.

The previous prime is 21691223. The next prime is 21691247. The reversal of 21691231 is 13219612.

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 21691231 - 2³ = 21691223 is a prime.

It is a super-2 number, since 2×21691231² = 941019004590722, 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 = 21691196 and 21691205.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21691531) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 471526 + ... + 471571.

It is an arithmetic number, because the mean of its divisors is an integer number (5658588).

Almost surely, 2^21691231 is an apocalyptic number.

21691231 is a deficient number, since it is larger than the sum of its proper divisors (943121).

21691231 is an equidigital number, since it uses as much as digits as its factorization.

21691231 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 943120.

The product of its digits is 648, while the sum is 25.

The square root of 21691231 is about 4657.3845664708. The cubic root of 21691231 is about 278.8868689122.

The spelling of 21691231 in words is "twenty-one million, six hundred ninety-one thousand, two hundred thirty-one".