21406721 has 8 divisors (see below), whose sum is σ = 25528704. Its totient is φ = 17550720.

The previous prime is 21406717. The next prime is 21406729. The reversal of 21406721 is 12760412.

21406721 = 80² + 81² + ... + 401².

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 21406721 - 2² = 21406717 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (23).

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21406729) 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, 66320 + ... + 66641.

It is an arithmetic number, because the mean of its divisors is an integer number (3191088).

Almost surely, 2^21406721 is an apocalyptic number.

It is an amenable number.

21406721 is a deficient number, since it is larger than the sum of its proper divisors (4121983).

21406721 is a wasteful number, since it uses less digits than its factorization.

21406721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 132991.

The product of its (nonzero) digits is 672, while the sum is 23.

The square root of 21406721 is about 4626.7397808824. The cubic root of 21406721 is about 277.6621717988.

The spelling of 21406721 in words is "twenty-one million, four hundred six thousand, seven hundred twenty-one".