21212705 has 16 divisors (see below), whose sum is σ = 26133840. Its totient is φ = 16524288.

The previous prime is 21212701. The next prime is 21212731. The reversal of 21212705 is 50721212.

It can be written as a sum of positive squares in 8 ways, for example, as 2411809 + 18800896 = 1553^2 + 4336^2 .

It is a cyclic number.

It is not a de Polignac number, because 21212705 - 2² = 21212701 is a prime.

It is an alternating number because its digits alternate between even and odd.

It is not an unprimeable number, because it can be changed into a prime (21212701) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 32159 + ... + 32811.

It is an arithmetic number, because the mean of its divisors is an integer number (1633365).

Almost surely, 2^21212705 is an apocalyptic number.

It is an amenable number.

21212705 is a deficient number, since it is larger than the sum of its proper divisors (4921135).

21212705 is an equidigital number, since it uses as much as digits as its factorization.

21212705 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 820.

The product of its (nonzero) digits is 280, while the sum is 20.

The square root of 21212705 is about 4605.7252414793. The cubic root of 21212705 is about 276.8207775133.

Adding to 21212705 its reverse (50721212), we get a palindrome (71933917).

The spelling of 21212705 in words is "twenty-one million, two hundred twelve thousand, seven hundred five".