21009408 has 88 divisors (see below), whose sum is σ = 64062912. Its totient is φ = 5996544.

The previous prime is 21009407. The next prime is 21009449. The reversal of 21009408 is 80490012.

It is a Harshad number since it is a multiple of its sum of digits (24).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21009403) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (5) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 21016 + ... + 21992.

Almost surely, 2^21009408 is an apocalyptic number.

21009408 is a gapful number since it is divisible by the number (28) formed by its first and last digit.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21009408, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (32031456).

21009408 is an abundant number, since it is smaller than the sum of its proper divisors (43053504).

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

21009408 is an equidigital number, since it uses as much as digits as its factorization.

21009408 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1007 (or 989 counting only the distinct ones).

The product of its (nonzero) digits is 576, while the sum is 24.

The square root of 21009408 is about 4583.6020769696. The cubic root of 21009408 is about 275.9336114215.

The spelling of 21009408 in words is "twenty-one million, nine thousand, four hundred eight".