21281720 has 32 divisors (see below), whose sum is σ = 48031920. Its totient is φ = 8486400.

The previous prime is 21281719. The next prime is 21281783. The reversal of 21281720 is 2718212.

It is a super-3 number, since 3×21281720³ (a number of 23 digits) contains 333 as substring.

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 7 ways as a sum of consecutive naturals, for example, 17120 + ... + 18320.

Almost surely, 2^21281720 is an apocalyptic number.

21281720 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21281720 is an abundant number, since it is smaller than the sum of its proper divisors (26750200).

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

21281720 is a wasteful number, since it uses less digits than its factorization.

21281720 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1655 (or 1651 counting only the distinct ones).

The product of its (nonzero) digits is 448, while the sum is 23.

The square root of 21281720 is about 4613.2114627448. The cubic root of 21281720 is about 277.1206623284.

Adding to 21281720 its reverse (2718212), we get a palindrome (23999932).

The spelling of 21281720 in words is "twenty-one million, two hundred eighty-one thousand, seven hundred twenty".