21688537 has 8 divisors (see below), whose sum is σ = 23416904. Its totient is φ = 19968768.

The previous prime is 21688531. The next prime is 21688543. The reversal of 21688537 is 73588612.

It is an interprime number because it is at equal distance from previous prime (21688531) and next prime (21688543).

It can be written as a sum of positive squares in 4 ways, for example, as 2143296 + 19545241 = 1464^2 + 4421^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 21688537 - 2⁷ = 21688409 is a prime.

It is a super-3 number, since 3×21688537³ (a number of 23 digits) contains 333 as substring.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21688537.

It is not an unprimeable number, because it can be changed into a prime (21688531) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 3703 + ... + 7555.

It is an arithmetic number, because the mean of its divisors is an integer number (2927113).

Almost surely, 2^21688537 is an apocalyptic number.

It is an amenable number.

21688537 is a deficient number, since it is larger than the sum of its proper divisors (1728367).

21688537 is a wasteful number, since it uses less digits than its factorization.

21688537 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4299.

The product of its digits is 80640, while the sum is 40.

The square root of 21688537 is about 4657.0953393720. The cubic root of 21688537 is about 278.8753227359.

The spelling of 21688537 in words is "twenty-one million, six hundred eighty-eight thousand, five hundred thirty-seven".