21688651 has 4 divisors (see below), whose sum is σ = 22964472. Its totient is φ = 20412832.

The previous prime is 21688649. The next prime is 21688661. The reversal of 21688651 is 15688612.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 21688651 - 2¹ = 21688649 is a prime.

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 21688651.

It is not an unprimeable number, because it can be changed into a prime (21688621) 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 3 ways as a sum of consecutive naturals, for example, 637885 + ... + 637918.

It is an arithmetic number, because the mean of its divisors is an integer number (5741118).

Almost surely, 2^21688651 is an apocalyptic number.

21688651 is a deficient number, since it is larger than the sum of its proper divisors (1275821).

21688651 is a wasteful number, since it uses less digits than its factorization.

21688651 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1275820.

The product of its digits is 23040, while the sum is 37.

The square root of 21688651 is about 4657.1075787446. The cubic root of 21688651 is about 278.8758113463.

The spelling of 21688651 in words is "twenty-one million, six hundred eighty-eight thousand, six hundred fifty-one".