21661645 has 8 divisors (see below), whose sum is σ = 26021880. Its totient is φ = 17310720.

The previous prime is 21661643. The next prime is 21661657. The reversal of 21661645 is 54616612.

It can be written as a sum of positive squares in 4 ways, for example, as 7327849 + 14333796 = 2707^2 + 3786^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 21661645 - 2¹ = 21661643 is a prime.

It is a super-3 number, since 3×21661645³ (a number of 23 digits) contains 333 as substring.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21661643) 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 7 ways as a sum of consecutive naturals, for example, 4765 + ... + 8125.

It is an arithmetic number, because the mean of its divisors is an integer number (3252735).

Almost surely, 2^21661645 is an apocalyptic number.

It is an amenable number.

21661645 is a deficient number, since it is larger than the sum of its proper divisors (4360235).

21661645 is a wasteful number, since it uses less digits than its factorization.

21661645 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4655.

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

The square root of 21661645 is about 4654.2072364690. The cubic root of 21661645 is about 278.7600142528.

The spelling of 21661645 in words is "twenty-one million, six hundred sixty-one thousand, six hundred forty-five".