21531609 has 16 divisors (see below), whose sum is σ = 34799040. Its totient is φ = 13049280.

The previous prime is 21531599. The next prime is 21531619. The reversal of 21531609 is 90613512.

21531609 is a `hidden beast` number, since 2 + 1 + 53 + 1 + 609 = 666.

It is an interprime number because it is at equal distance from previous prime (21531599) and next prime (21531619).

It is not a de Polignac number, because 21531609 - 2⁷ = 21531481 is a prime.

It is a super-2 number, since 2×21531609² = 927220372257762, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21531619) 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, 35952 + ... + 36545.

It is an arithmetic number, because the mean of its divisors is an integer number (2174940).

Almost surely, 2^21531609 is an apocalyptic number.

It is an amenable number.

21531609 is a deficient number, since it is larger than the sum of its proper divisors (13267431).

21531609 is a wasteful number, since it uses less digits than its factorization.

21531609 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 72517 (or 72511 counting only the distinct ones).

The product of its (nonzero) digits is 1620, while the sum is 27.

The square root of 21531609 is about 4640.2164820189. The cubic root of 21531609 is about 278.2010906399.

The spelling of 21531609 in words is "twenty-one million, five hundred thirty-one thousand, six hundred nine".