21158509 has 4 divisors (see below), whose sum is σ = 21170304. Its totient is φ = 21146716.

The previous prime is 21158471. The next prime is 21158519. The reversal of 21158509 is 90585112.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also a brilliant number, because the two primes have the same length.

It is a cyclic number.

It is not a de Polignac number, because 21158509 - 2¹⁹ = 20634221 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21158519) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2587 + ... + 7000.

It is an arithmetic number, because the mean of its divisors is an integer number (5292576).

Almost surely, 2^21158509 is an apocalyptic number.

It is an amenable number.

21158509 is a deficient number, since it is larger than the sum of its proper divisors (11795).

21158509 is an equidigital number, since it uses as much as digits as its factorization.

21158509 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 11794.

The product of its (nonzero) digits is 3600, while the sum is 31.

The square root of 21158509 is about 4599.8379319276. The cubic root of 21158509 is about 276.5848281355.

It can be divided in two parts, 2 and 1158509, that added together give a palindrome (1158511).

The spelling of 21158509 in words is "twenty-one million, one hundred fifty-eight thousand, five hundred nine".