21687309 has 12 divisors (see below), whose sum is σ = 35801376. Its totient is φ = 12392712.

The previous prime is 21687307. The next prime is 21687329. The reversal of 21687309 is 90378612.

It is not a de Polignac number, because 21687309 - 2¹ = 21687307 is a prime.

It is a super-2 number, since 2×21687309² = 940678743322962, which contains 22 as substring.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21687301) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 172059 + ... + 172184.

It is an arithmetic number, because the mean of its divisors is an integer number (2983448).

Almost surely, 2^21687309 is an apocalyptic number.

It is an amenable number.

21687309 is a deficient number, since it is larger than the sum of its proper divisors (14114067).

21687309 is a wasteful number, since it uses less digits than its factorization.

21687309 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 344256 (or 344253 counting only the distinct ones).

The product of its (nonzero) digits is 18144, while the sum is 36.

The square root of 21687309 is about 4656.9634956697. The cubic root of 21687309 is about 278.8700593509.

The spelling of 21687309 in words is "twenty-one million, six hundred eighty-seven thousand, three hundred nine".