21871720 has 32 divisors (see below), whose sum is σ = 52998120. Its totient is φ = 8075520.

The previous prime is 21871667. The next prime is 21871721. The reversal of 21871720 is 2717812.

It can be written as a sum of positive squares in 4 ways, for example, as 20016676 + 1855044 = 4474^2 + 1362^2 .

It is a Smith number, since the sum of its digits (28) coincides with the sum of the digits of its prime factors.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21871721) 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 7 ways as a sum of consecutive naturals, for example, 20511 + ... + 21550.

Almost surely, 2^21871720 is an apocalyptic number.

21871720 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21871720 is an abundant number, since it is smaller than the sum of its proper divisors (31126400).

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

21871720 is a wasteful number, since it uses less digits than its factorization.

21871720 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 42085 (or 42081 counting only the distinct ones).

The product of its (nonzero) digits is 1568, while the sum is 28.

The square root of 21871720 is about 4676.7210735728. The cubic root of 21871720 is about 279.6582565370.

The spelling of 21871720 in words is "twenty-one million, eight hundred seventy-one thousand, seven hundred twenty".