21438509 has 4 divisors (see below), whose sum is σ = 21456060. Its totient is φ = 21420960.

The previous prime is 21438499. The next prime is 21438541. The reversal of 21438509 is 90583412.

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 + 990025 = 4522^2 + 995^2 .

It is a cyclic number.

It is not a de Polignac number, because 21438509 - 2⁸ = 21438253 is a prime.

It is an alternating number because its digits alternate between even and odd.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21438589) 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, 6794 + ... + 9435.

It is an arithmetic number, because the mean of its divisors is an integer number (5364015).

Almost surely, 2^21438509 is an apocalyptic number.

It is an amenable number.

21438509 is a deficient number, since it is larger than the sum of its proper divisors (17551).

21438509 is a wasteful number, since it uses less digits than its factorization.

21438509 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 17550.

The product of its (nonzero) digits is 8640, while the sum is 32.

The square root of 21438509 is about 4630.1737548390. The cubic root of 21438509 is about 277.7995423598.

The spelling of 21438509 in words is "twenty-one million, four hundred thirty-eight thousand, five hundred nine".