21518609 has 16 divisors (see below), whose sum is σ = 25816320. Its totient is φ = 17545920.

The previous prime is 21518603. The next prime is 21518611. The reversal of 21518609 is 90681512.

It is a de Polignac number, because none of the positive numbers 2^k-21518609 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21518603) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 13667 + ... + 15159.

It is an arithmetic number, because the mean of its divisors is an integer number (1613520).

Almost surely, 2^21518609 is an apocalyptic number.

21518609 is a gapful number since it is divisible by the number (29) formed by its first and last digit.

It is an amenable number.

21518609 is a deficient number, since it is larger than the sum of its proper divisors (4297711).

21518609 is a wasteful number, since it uses less digits than its factorization.

21518609 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1600.

The product of its (nonzero) digits is 4320, while the sum is 32.

The square root of 21518609 is about 4638.8154738036. The cubic root of 21518609 is about 278.1450901449.

The spelling of 21518609 in words is "twenty-one million, five hundred eighteen thousand, six hundred nine".