21011220 has 72 divisors (see below), whose sum is σ = 64360296. Its totient is φ = 5548032.

The previous prime is 21011191. The next prime is 21011231. The reversal of 21011220 is 2211012.

It can be written as a sum of positive squares in 4 ways, for example, as 1774224 + 19236996 = 1332^2 + 4386^2 .

It is a Harshad number since it is a multiple of its sum of digits (9).

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 19824 + ... + 20856.

It is an arithmetic number, because the mean of its divisors is an integer number (893893).

Almost surely, 2^21011220 is an apocalyptic number.

21011220 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 21011220, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (32180148).

21011220 is an abundant number, since it is smaller than the sum of its proper divisors (43349076).

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

21011220 is a wasteful number, since it uses less digits than its factorization.

21011220 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1161 (or 1156 counting only the distinct ones).

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

The square root of 21011220 is about 4583.7997338453. The cubic root of 21011220 is about 275.9415440158.

Adding to 21011220 its reverse (2211012), we get a palindrome (23222232).

It can be divided in two parts, 2101 and 1220, that added together give a triangular number (3321 = T₈₁).

The spelling of 21011220 in words is "twenty-one million, eleven thousand, two hundred twenty".