21687648 has 96 divisors (see below), whose sum is σ = 61344864. Its totient is φ = 6684672.

The previous prime is 21687641. The next prime is 21687691. The reversal of 21687648 is 84678612.

It is a Cunningham number, because it is equal to 4657²-1.

It is a tau number, because it is divible by the number of its divisors (96).

Its product of digits (129024) is a multiple of the sum of its prime divisors (256).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21687641) 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, 158236 + ... + 158372.

It is an arithmetic number, because the mean of its divisors is an integer number (639009).

Almost surely, 2^21687648 is an apocalyptic number.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21687648, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (30672432).

21687648 is an abundant number, since it is smaller than the sum of its proper divisors (39657216).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

21687648 is a wasteful number, since it uses less digits than its factorization.

21687648 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 264 (or 256 counting only the distinct ones).

The product of its digits is 129024, while the sum is 42.

The square root of 21687648 is about 4656.9998926347. Note that the first 3 decimals coincide. The cubic root of 21687648 is about 278.8715123736.

The spelling of 21687648 in words is "twenty-one million, six hundred eighty-seven thousand, six hundred forty-eight".