21621816 has 80 divisors (see below), whose sum is σ = 61666440. Its totient is φ = 7076160.

The previous prime is 21621793. The next prime is 21621823. The reversal of 21621816 is 61812612.

21621816 is a `hidden beast` number, since 21 + 621 + 8 + 16 = 666.

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

It is a nude number because it is divisible by every one of its digits.

It is a zygodrome in base 11.

It is a self number, because there is not a number n which added to its sum of digits gives 21621816.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 19 ways as a sum of consecutive naturals, for example, 39255 + ... + 39801.

Almost surely, 2^21621816 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 21621816, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (30833220).

21621816 is an abundant number, since it is smaller than the sum of its proper divisors (40044624).

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

21621816 is a wasteful number, since it uses less digits than its factorization.

21621816 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 626 (or 613 counting only the distinct ones).

The product of its digits is 1152, while the sum is 27.

The square root of 21621816 is about 4649.9264510312. The cubic root of 21621816 is about 278.5890585630.

The spelling of 21621816 in words is "twenty-one million, six hundred twenty-one thousand, eight hundred sixteen".