21709728 has 48 divisors (see below), whose sum is σ = 63322560. Its totient is φ = 7236288.

The previous prime is 21709693. The next prime is 21709733. The reversal of 21709728 is 82790712.

It is a tau number, because it is divible by the number of its divisors (48).

It is a Smith number, since the sum of its digits (36) coincides with the sum of the digits of its prime factors.

It is a Harshad number since it is a multiple of its sum of digits (36).

It is a junction number, because it is equal to n+sod(n) for n = 21709692 and 21709701.

It is an unprimeable number.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 11700 + ... + 13427.

It is an arithmetic number, because the mean of its divisors is an integer number (1319220).

Almost surely, 2^21709728 is an apocalyptic number.

It is an amenable number.

21709728 is an abundant number, since it is smaller than the sum of its proper divisors (41612832).

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

21709728 is a wasteful number, since it uses less digits than its factorization.

21709728 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 25146 (or 25132 counting only the distinct ones).

The product of its (nonzero) digits is 14112, while the sum is 36.

The square root of 21709728 is about 4659.3699144841. The cubic root of 21709728 is about 278.9661191370.

The spelling of 21709728 in words is "twenty-one million, seven hundred nine thousand, seven hundred twenty-eight".