21216013566 has 64 divisors (see below), whose sum is σ = 49079105280. Its totient is φ = 6042677760.

The previous prime is 21216013547. The next prime is 21216013571. The reversal of 21216013566 is 66531061212.

It is a Harshad number since it is a multiple of its sum of digits (33).

It is a congruent number.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 836865 + ... + 861843.

It is an arithmetic number, because the mean of its divisors is an integer number (766861020).

Almost surely, 2^21216013566 is an apocalyptic number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21216013566, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (24539552640).

21216013566 is an abundant number, since it is smaller than the sum of its proper divisors (27863091714).

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

21216013566 is a wasteful number, since it uses less digits than its factorization.

21216013566 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 25769.

The product of its (nonzero) digits is 12960, while the sum is 33.

Adding to 21216013566 its reverse (66531061212), we get a palindrome (87747074778).

The spelling of 21216013566 in words is "twenty-one billion, two hundred sixteen million, thirteen thousand, five hundred sixty-six".