21991543 has 6 divisors (see below), whose sum is σ = 25582056. Its totient is φ = 18849852.

The previous prime is 21991517. The next prime is 21991561. The reversal of 21991543 is 34519912.

It is not a de Polignac number, because 21991543 - 2¹⁷ = 21860471 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21991499 and 21991508.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21991573) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 224355 + ... + 224452.

It is an arithmetic number, because the mean of its divisors is an integer number (4263676).

Almost surely, 2^21991543 is an apocalyptic number.

21991543 is a deficient number, since it is larger than the sum of its proper divisors (3590513).

21991543 is an equidigital number, since it uses as much as digits as its factorization.

21991543 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 448821 (or 448814 counting only the distinct ones).

The product of its digits is 9720, while the sum is 34.

The square root of 21991543 is about 4689.5141539396. The cubic root of 21991543 is about 280.1680241508.

The spelling of 21991543 in words is "twenty-one million, nine hundred ninety-one thousand, five hundred forty-three".