21081816 has 128 divisors (see below), whose sum is σ = 68198400. Its totient is φ = 5909760.

The previous prime is 21081793. The next prime is 21081833. The reversal of 21081816 is 61818012.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is a self number, because there is not a number n which added to its sum of digits gives 21081816.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 110281 + ... + 110471.

It is an arithmetic number, because the mean of its divisors is an integer number (532800).

Almost surely, 2^21081816 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 21081816, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (34099200).

21081816 is an abundant number, since it is smaller than the sum of its proper divisors (47116584).

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

21081816 is a wasteful number, since it uses less digits than its factorization.

21081816 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 286 (or 276 counting only the distinct ones).

The product of its (nonzero) digits is 768, while the sum is 27.

The square root of 21081816 is about 4591.4938745467. The cubic root of 21081816 is about 276.2502456267.

Adding to 21081816 its reverse (61818012), we get a palindrome (82899828).

The spelling of 21081816 in words is "twenty-one million, eighty-one thousand, eight hundred sixteen".