21721065 has 16 divisors (see below), whose sum is σ = 35274048. Its totient is φ = 11411136.

The previous prime is 21721057. The next prime is 21721067. The reversal of 21721065 is 56012712.

It is not a de Polignac number, because 21721065 - 2³ = 21721057 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21721067) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 9802 + ... + 11811.

It is an arithmetic number, because the mean of its divisors is an integer number (2204628).

Almost surely, 2^21721065 is an apocalyptic number.

It is an amenable number.

21721065 is a deficient number, since it is larger than the sum of its proper divisors (13552983).

21721065 is a wasteful number, since it uses less digits than its factorization.

21721065 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 21688.

The product of its (nonzero) digits is 840, while the sum is 24.

The square root of 21721065 is about 4660.5863365032. The cubic root of 21721065 is about 279.0146701604.

Adding to 21721065 its reverse (56012712), we get a palindrome (77733777).

The spelling of 21721065 in words is "twenty-one million, seven hundred twenty-one thousand, sixty-five".