21286721 has 4 divisors (see below), whose sum is σ = 21307392. Its totient is φ = 21266052.

The previous prime is 21286717. The next prime is 21286751. The reversal of 21286721 is 12768212.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 21286721 - 2² = 21286717 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21286751) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 8705 + ... + 10878.

It is an arithmetic number, because the mean of its divisors is an integer number (5326848).

Almost surely, 2^21286721 is an apocalyptic number.

It is an amenable number.

21286721 is a deficient number, since it is larger than the sum of its proper divisors (20671).

21286721 is a wasteful number, since it uses less digits than its factorization.

21286721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 20670.

The product of its digits is 2688, while the sum is 29.

The square root of 21286721 is about 4613.7534611203. The cubic root of 21286721 is about 277.1423675272.

The spelling of 21286721 in words is "twenty-one million, two hundred eighty-six thousand, seven hundred twenty-one".