21781228 has 12 divisors (see below), whose sum is σ = 43562512. Its totient is φ = 9334800.

The previous prime is 21781223. The next prime is 21781247. The reversal of 21781228 is 82218712.

21781228 is an admirable number.

It is a self number, because there is not a number n which added to its sum of digits gives 21781228.

It is not an unprimeable number, because it can be changed into a prime (21781223) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 388923 + ... + 388978.

Almost surely, 2^21781228 is an apocalyptic number.

21781228 is a gapful number since it is divisible by the number (28) formed by its first and last digit.

It is an amenable number.

21781228 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.

21781228 is a wasteful number, since it uses less digits than its factorization.

21781228 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 777912 (or 777910 counting only the distinct ones).

The product of its digits is 3584, while the sum is 31.

The square root of 21781228 is about 4667.0363186931. The cubic root of 21781228 is about 279.2720376072.

The spelling of 21781228 in words is "twenty-one million, seven hundred eighty-one thousand, two hundred twenty-eight".