21217428 has 72 divisors (see below), whose sum is σ = 58384872. Its totient is φ = 6469632.

The previous prime is 21217421. The next prime is 21217453. The reversal of 21217428 is 82471212.

21217428 is a `hidden beast` number, since 21 + 217 + 428 = 666.

It can be written as a sum of positive squares in 4 ways, for example, as 4717584 + 16499844 = 2172^2 + 4062^2 .

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21217421) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 22176 + ... + 23112.

It is an arithmetic number, because the mean of its divisors is an integer number (810901).

Almost surely, 2^21217428 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 21217428, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (29192436).

21217428 is an abundant number, since it is smaller than the sum of its proper divisors (37167444).

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

21217428 is a wasteful number, since it uses less digits than its factorization.

21217428 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1001 (or 996 counting only the distinct ones).

The product of its digits is 1792, while the sum is 27.

The square root of 21217428 is about 4606.2379443533. The cubic root of 21217428 is about 276.8413206660.

The spelling of 21217428 in words is "twenty-one million, two hundred seventeen thousand, four hundred twenty-eight".