21987509 has 6 divisors (see below), whose sum is σ = 22599234. Its totient is φ = 21391920.

The previous prime is 21987487. The next prime is 21987523. The reversal of 21987509 is 90578912.

It can be written as a sum of positive squares in 3 ways, for example, as 5152900 + 16834609 = 2270^2 + 4103^2 .

It is not a de Polignac number, because 21987509 - 2¹² = 21983413 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 (21987529) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 6662 + ... + 9399.

It is an arithmetic number, because the mean of its divisors is an integer number (3766539).

Almost surely, 2^21987509 is an apocalyptic number.

It is an amenable number.

21987509 is a deficient number, since it is larger than the sum of its proper divisors (611725).

21987509 is an equidigital number, since it uses as much as digits as its factorization.

21987509 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 16135 (or 16098 counting only the distinct ones).

The product of its (nonzero) digits is 45360, while the sum is 41.

The square root of 21987509 is about 4689.0840256920. The cubic root of 21987509 is about 280.1508923088.

The spelling of 21987509 in words is "twenty-one million, nine hundred eighty-seven thousand, five hundred nine".