21878509 has 4 divisors (see below), whose sum is σ = 21888316. Its totient is φ = 21868704.

The previous prime is 21878491. The next prime is 21878531. The reversal of 21878509 is 90587812.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It can be written as a sum of positive squares in 2 ways, for example, as 12938409 + 8940100 = 3597^2 + 2990^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21878509 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 (21878569) 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, 247 + ... + 6619.

It is an arithmetic number, because the mean of its divisors is an integer number (5472079).

Almost surely, 2^21878509 is an apocalyptic number.

It is an amenable number.

21878509 is a deficient number, since it is larger than the sum of its proper divisors (9807).

21878509 is an equidigital number, since it uses as much as digits as its factorization.

21878509 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 9806.

The product of its (nonzero) digits is 40320, while the sum is 40.

The square root of 21878509 is about 4677.4468463041. The cubic root of 21878509 is about 279.6871889284.

The spelling of 21878509 in words is "twenty-one million, eight hundred seventy-eight thousand, five hundred nine".