21721145 has 16 divisors (see below), whose sum is σ = 27432000. Its totient is φ = 16486848.

The previous prime is 21721127. The next prime is 21721151. The reversal of 21721145 is 54112712.

It is not a de Polignac number, because 21721145 - 2⁶ = 21721081 is a prime.

It is a super-2 number, since 2×21721145² = 943616280222050, which contains 22 as substring.

It is a self number, because there is not a number n which added to its sum of digits gives 21721145.

It is an unprimeable number.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 7286 + ... + 9824.

It is an arithmetic number, because the mean of its divisors is an integer number (1714500).

Almost surely, 2^21721145 is an apocalyptic number.

It is an amenable number.

21721145 is a deficient number, since it is larger than the sum of its proper divisors (5710855).

21721145 is a wasteful number, since it uses less digits than its factorization.

21721145 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2632.

The product of its digits is 560, while the sum is 23.

The square root of 21721145 is about 4660.5949191064. The cubic root of 21721145 is about 279.0150127026.

Adding to 21721145 its reverse (54112712), we get a palindrome (75833857).

The spelling of 21721145 in words is "twenty-one million, seven hundred twenty-one thousand, one hundred forty-five".