21399840 has 288 divisors, whose sum is σ = 91518336. Its totient is φ = 4423680.

The previous prime is 21399839. The next prime is 21399901. The reversal of 21399840 is 4899312.

It is a tau number, because it is divible by the number of its divisors (288).

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 = 21399795 and 21399804.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 47 ways as a sum of consecutive naturals, for example, 110784 + ... + 110976.

It is an arithmetic number, because the mean of its divisors is an integer number (317772).

Almost surely, 2^21399840 is an apocalyptic number.

21399840 is a gapful number since it is divisible by the number (20) 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 21399840, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (45759168).

21399840 is an abundant number, since it is smaller than the sum of its proper divisors (70118496).

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

21399840 is a wasteful number, since it uses less digits than its factorization.

21399840 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 232 (or 221 counting only the distinct ones).

The product of its (nonzero) digits is 15552, while the sum is 36.

The square root of 21399840 is about 4625.9961089478. The cubic root of 21399840 is about 277.6324179323.

Adding to 21399840 its sum of digits (36), we get a square (21399876 = 4626²).

The spelling of 21399840 in words is "twenty-one million, three hundred ninety-nine thousand, eight hundred forty".