21450721 has 4 divisors (see below), whose sum is σ = 21467844. Its totient is φ = 21433600.

The previous prime is 21450707. The next prime is 21450727. The reversal of 21450721 is 12705412.

It is a happy number.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 8346321 + 13104400 = 2889^2 + 3620^2 .

It is a cyclic number.

It is not a de Polignac number, because 21450721 - 2¹⁵ = 21417953 is a prime.

It is an alternating number because its digits alternate between even and odd.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21450692 and 21450701.

It is not an unprimeable number, because it can be changed into a prime (21450727) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 6520 + ... + 9241.

It is an arithmetic number, because the mean of its divisors is an integer number (5366961).

Almost surely, 2^21450721 is an apocalyptic number.

It is an amenable number.

21450721 is a deficient number, since it is larger than the sum of its proper divisors (17123).

21450721 is a wasteful number, since it uses less digits than its factorization.

21450721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 17122.

The product of its (nonzero) digits is 560, while the sum is 22.

The square root of 21450721 is about 4631.4923081011. The cubic root of 21450721 is about 277.8522799213.

The spelling of 21450721 in words is "twenty-one million, four hundred fifty thousand, seven hundred twenty-one".