21513509 has 4 divisors (see below), whose sum is σ = 21522816. Its totient is φ = 21504204.

The previous prime is 21513469. The next prime is 21513511. The reversal of 21513509 is 90531512.

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 21513509 - 2⁸ = 21513253 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 (21513559) 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, 1772 + ... + 6794.

It is an arithmetic number, because the mean of its divisors is an integer number (5380704).

Almost surely, 2^21513509 is an apocalyptic number.

It is an amenable number.

21513509 is a deficient number, since it is larger than the sum of its proper divisors (9307).

21513509 is an equidigital number, since it uses as much as digits as its factorization.

21513509 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 9306.

The product of its (nonzero) digits is 1350, while the sum is 26.

The square root of 21513509 is about 4638.2657319304. The cubic root of 21513509 is about 278.1231145603.

It can be divided in two parts, 21513 and 509, that added together give a palindrome (22022).

The spelling of 21513509 in words is "twenty-one million, five hundred thirteen thousand, five hundred nine".