21100221 has 6 divisors (see below), whose sum is σ = 30478110. Its totient is φ = 14066808.

The previous prime is 21100217. The next prime is 21100231. The reversal of 21100221 is 12200112.

It can be written as a sum of positive squares in only one way, i.e., 19236996 + 1863225 = 4386^2 + 1365^2 .

It is not a de Polignac number, because 21100221 - 2² = 21100217 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (9), and also a Moran number because the ratio is a prime number: 2344469 = 21100221 / (2 + 1 + 1 + 0 + 0 + 2 + 2 + 1).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21100231) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 1172226 + ... + 1172243.

It is an arithmetic number, because the mean of its divisors is an integer number (5079685).

Almost surely, 2^21100221 is an apocalyptic number.

It is an amenable number.

21100221 is a deficient number, since it is larger than the sum of its proper divisors (9377889).

21100221 is a wasteful number, since it uses less digits than its factorization.

21100221 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2344475 (or 2344472 counting only the distinct ones).

The product of its (nonzero) digits is 8, while the sum is 9.

The square root of 21100221 is about 4593.4976869484. The cubic root of 21100221 is about 276.3306135749.

Adding to 21100221 its reverse (12200112), we get a palindrome (33300333).

It can be divided in two parts, 21100 and 221, that added together give a triangular number (21321 = T₂₀₆).

The spelling of 21100221 in words is "twenty-one million, one hundred thousand, two hundred twenty-one".