21131220 has 96 divisors (see below), whose sum is σ = 65390976. Its totient is φ = 5056000.

The previous prime is 21131191. The next prime is 21131251. The reversal of 21131220 is 2213112.

It is a Harshad number since it is a multiple of its sum of digits (12).

It is a junction number, because it is equal to n+sod(n) for n = 21131196 and 21131205.

It is a congruent number.

It is an unprimeable number.

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 31 ways as a sum of consecutive naturals, for example, 66502 + ... + 66818.

It is an arithmetic number, because the mean of its divisors is an integer number (681156).

Almost surely, 2^21131220 is an apocalyptic number.

21131220 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 21131220, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (32695488).

21131220 is an abundant number, since it is smaller than the sum of its proper divisors (44259756).

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

21131220 is a wasteful number, since it uses less digits than its factorization.

21131220 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 441 (or 439 counting only the distinct ones).

The product of its (nonzero) digits is 24, while the sum is 12.

The square root of 21131220 is about 4596.8706747090. The cubic root of 21131220 is about 276.4658693662.

Adding to 21131220 its reverse (2213112), we get a palindrome (23344332).

It can be divided in two parts, 2113 and 1220, that added together give a palindrome (3333).

The spelling of 21131220 in words is "twenty-one million, one hundred thirty-one thousand, two hundred twenty".