21000805 has 16 divisors (see below), whose sum is σ = 29001600. Its totient is φ = 14300352.

The previous prime is 21000803. The next prime is 21000821. The reversal of 21000805 is 50800012.

It is a happy number.

It is a cyclic number.

It is not a de Polignac number, because 21000805 - 2¹ = 21000803 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21000803) 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, 3202 + ... + 7228.

It is an arithmetic number, because the mean of its divisors is an integer number (1812600).

Almost surely, 2^21000805 is an apocalyptic number.

It is an amenable number.

21000805 is a deficient number, since it is larger than the sum of its proper divisors (8000795).

21000805 is a wasteful number, since it uses less digits than its factorization.

21000805 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4188.

The product of its (nonzero) digits is 80, while the sum is 16.

The square root of 21000805 is about 4582.6635268149. The cubic root of 21000805 is about 275.8959428851.

Adding to 21000805 its reverse (50800012), we get a palindrome (71800817).

The spelling of 21000805 in words is "twenty-one million, eight hundred five", and thus it is an aban number.