21561509 has 4 divisors (see below), whose sum is σ = 21686316. Its totient is φ = 21436704.

The previous prime is 21561503. The next prime is 21561571. The reversal of 21561509 is 90516512.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 20448484 + 1113025 = 4522^2 + 1055^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21561509 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 (21561503) 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, 62144 + ... + 62489.

It is an arithmetic number, because the mean of its divisors is an integer number (5421579).

Almost surely, 2^21561509 is an apocalyptic number.

It is an amenable number.

21561509 is a deficient number, since it is larger than the sum of its proper divisors (124807).

21561509 is a wasteful number, since it uses less digits than its factorization.

21561509 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 124806.

The product of its (nonzero) digits is 2700, while the sum is 29.

The square root of 21561509 is about 4643.4371967326. The cubic root of 21561509 is about 278.3298062907.

The spelling of 21561509 in words is "twenty-one million, five hundred sixty-one thousand, five hundred nine".