21561100 has 72 divisors (see below), whose sum is σ = 54090288. Its totient is φ = 7372800.

The previous prime is 21561091. The next prime is 21561101. The reversal of 21561100 is 116512.

It is a happy number.

It is not an unprimeable number, because it can be changed into a prime (21561101) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 18124 + ... + 19276.

It is an arithmetic number, because the mean of its divisors is an integer number (751254).

Almost surely, 2^21561100 is an apocalyptic number.

21561100 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 21561100, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (27045144).

21561100 is an abundant number, since it is smaller than the sum of its proper divisors (32529188).

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

21561100 is a wasteful number, since it uses less digits than its factorization.

21561100 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1195 (or 1188 counting only the distinct ones).

The product of its (nonzero) digits is 60, while the sum is 16.

The square root of 21561100 is about 4643.3931558721. The cubic root of 21561100 is about 278.3280464014.

Adding to 21561100 its reverse (116512), we get a palindrome (21677612).

It can be divided in two parts, 2156 and 1100, that multiplied together give a square (2371600 = 1540²).

The spelling of 21561100 in words is "twenty-one million, five hundred sixty-one thousand, one hundred".