21720001 has 4 divisors (see below), whose sum is σ = 21876400. Its totient is φ = 21563604.

The previous prime is 21719989. The next prime is 21720007. The reversal of 21720001 is 10002712.

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 21720001 - 2¹¹ = 21717953 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21720007) by changing a digit.

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 3 ways as a sum of consecutive naturals, for example, 77991 + ... + 78268.

It is an arithmetic number, because the mean of its divisors is an integer number (5469100).

Almost surely, 2^21720001 is an apocalyptic number.

It is an amenable number.

21720001 is a deficient number, since it is larger than the sum of its proper divisors (156399).

21720001 is a wasteful number, since it uses less digits than its factorization.

21720001 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 156398.

The product of its (nonzero) digits is 28, while the sum is 13.

The square root of 21720001 is about 4660.4721863777. The cubic root of 21720001 is about 279.0101142687.

Adding to 21720001 its reverse (10002712), we get a palindrome (31722713).

The spelling of 21720001 in words is "twenty-one million, seven hundred twenty thousand, one".