21006643 has 12 divisors (see below), whose sum is σ = 25280640. Its totient is φ = 17383632.

The previous prime is 21006641. The next prime is 21006673. The reversal of 21006643 is 34660012.

It is not a de Polignac number, because 21006643 - 2¹ = 21006641 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21006641) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 5971 + ... + 8812.

It is an arithmetic number, because the mean of its divisors is an integer number (2106720).

Almost surely, 2^21006643 is an apocalyptic number.

21006643 is a deficient number, since it is larger than the sum of its proper divisors (4273997).

21006643 is a wasteful number, since it uses less digits than its factorization.

21006643 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 14826 (or 14819 counting only the distinct ones).

The product of its (nonzero) digits is 864, while the sum is 22.

The square root of 21006643 is about 4583.3004483669. The cubic root of 21006643 is about 275.9215058938.

Adding to 21006643 its reverse (34660012), we get a palindrome (55666655).

The spelling of 21006643 in words is "twenty-one million, six thousand, six hundred forty-three".