21131720 has 32 divisors (see below), whose sum is σ = 49188600. Its totient is φ = 8160768.

The previous prime is 21131713. The next prime is 21131723. The reversal of 21131720 is 2713112.

It can be written as a sum of positive squares in 4 ways, for example, as 14485636 + 6646084 = 3806^2 + 2578^2 .

It is not an unprimeable number, because it can be changed into a prime (21131723) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 7949 + ... + 10268.

Almost surely, 2^21131720 is an apocalyptic number.

21131720 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21131720 is an abundant number, since it is smaller than the sum of its proper divisors (28056880).

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

21131720 is a wasteful number, since it uses less digits than its factorization.

21131720 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 18257 (or 18253 counting only the distinct ones).

The product of its (nonzero) digits is 84, while the sum is 17.

The square root of 21131720 is about 4596.9250592108. The cubic root of 21131720 is about 276.4680498973.

Adding to 21131720 its reverse (2713112), we get a palindrome (23844832).

The spelling of 21131720 in words is "twenty-one million, one hundred thirty-one thousand, seven hundred twenty".