21721328 has 20 divisors (see below), whose sum is σ = 43443648. Its totient is φ = 10510080.

The previous prime is 21721303. The next prime is 21721373. The reversal of 21721328 is 82312712.

21721328 is an admirable number.

It is a junction number, because it is equal to n+sod(n) for n = 21721297 and 21721306.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 21401 + ... + 22392.

Almost surely, 2^21721328 is an apocalyptic number.

It is an amenable number.

21721328 is a primitive abundant number, since it is smaller than the sum of its proper divisors, none of which is abundant.

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

21721328 is a wasteful number, since it uses less digits than its factorization.

21721328 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 43832 (or 43826 counting only the distinct ones).

The product of its digits is 1344, while the sum is 26.

The square root of 21721328 is about 4660.6145517517. The cubic root of 21721328 is about 279.0157962648.

The spelling of 21721328 in words is "twenty-one million, seven hundred twenty-one thousand, three hundred twenty-eight".