21616645 has 4 divisors (see below), whose sum is σ = 25939980. Its totient is φ = 17293312.

The previous prime is 21616643. The next prime is 21616657. The reversal of 21616645 is 54661612.

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 7001316 + 14615329 = 2646^2 + 3823^2 .

It is a cyclic number.

It is not a de Polignac number, because 21616645 - 2¹ = 21616643 is a prime.

It is a Smith number, since the sum of its digits (31) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21616643) 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, 2161660 + ... + 2161669.

It is an arithmetic number, because the mean of its divisors is an integer number (6484995).

Almost surely, 2^21616645 is an apocalyptic number.

It is an amenable number.

21616645 is a deficient number, since it is larger than the sum of its proper divisors (4323335).

21616645 is an equidigital number, since it uses as much as digits as its factorization.

21616645 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4323334.

The product of its digits is 8640, while the sum is 31.

The square root of 21616645 is about 4649.3703874826. The cubic root of 21616645 is about 278.5668479883.

It can be divided in two parts, 216 and 16645, that added together give a palindrome (16861).

The spelling of 21616645 in words is "twenty-one million, six hundred sixteen thousand, six hundred forty-five".