21687649 has 3 divisors (see below), whose sum is σ = 21692307. Its totient is φ = 21682992.

The previous prime is 21687641. The next prime is 21687691. The reversal of 21687649 is 94678612.

The square root of 21687649 is 4657.

It is a perfect power (a square), and thus also a powerful number.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It can be written as a sum of positive squares in only one way, i.e., 2608225 + 19079424 = 1615^2 + 4368^2 .

It is not a de Polignac number, because 21687649 - 2³ = 21687641 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21687641) 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 2 ways as a sum of consecutive naturals, for example, 2329 + ... + 6985.

It is an arithmetic number, because the mean of its divisors is an integer number (7230769).

Almost surely, 2^21687649 is an apocalyptic number.

21687649 is the 4657-th square number.

21687649 is the 2329-th centered octagonal number.

It is an amenable number.

21687649 is a deficient number, since it is larger than the sum of its proper divisors (4658).

21687649 is an frugal number, since it uses more digits than its factorization.

21687649 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 9314 (or 4657 counting only the distinct ones).

The product of its digits is 145152, while the sum is 43.

The cubic root of 21687649 is about 278.8715166598.

The spelling of 21687649 in words is "twenty-one million, six hundred eighty-seven thousand, six hundred forty-nine".