21681536 has 32 divisors (see below), whose sum is σ = 43605000. Its totient is φ = 10737664.

The previous prime is 21681533. The next prime is 21681547. The reversal of 21681536 is 63518612.

It is a happy number.

It is a tau number, because it is divible by the number of its divisors (32).

It is a Harshad number since it is a multiple of its sum of digits (32).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21681533) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 13715 + ... + 15213.

Almost surely, 2^21681536 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 21681536, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (21802500).

21681536 is an abundant number, since it is smaller than the sum of its proper divisors (21923464).

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

21681536 is a wasteful number, since it uses less digits than its factorization.

21681536 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1626 (or 1614 counting only the distinct ones).

The product of its digits is 8640, while the sum is 32.

The square root of 21681536 is about 4656.3436299311. The cubic root of 21681536 is about 278.8453127780.

The spelling of 21681536 in words is "twenty-one million, six hundred eighty-one thousand, five hundred thirty-six".