21509729 has 4 divisors (see below), whose sum is σ = 22641840. Its totient is φ = 20377620.

The previous prime is 21509723. The next prime is 21509731. The reversal of 21509729 is 92790512.

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.

It is a cyclic number.

It is not a de Polignac number, because 21509729 - 2⁴ = 21509713 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21509723) 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, 566027 + ... + 566064.

It is an arithmetic number, because the mean of its divisors is an integer number (5660460).

Almost surely, 2^21509729 is an apocalyptic number.

It is an amenable number.

21509729 is a deficient number, since it is larger than the sum of its proper divisors (1132111).

21509729 is a wasteful number, since it uses less digits than its factorization.

21509729 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1132110.

The product of its (nonzero) digits is 11340, while the sum is 35.

The square root of 21509729 is about 4637.8582341421. The cubic root of 21509729 is about 278.1068245329.

The spelling of 21509729 in words is "twenty-one million, five hundred nine thousand, seven hundred twenty-nine".