21945809 has 2 divisors, whose sum is σ = 21945810. Its totient is φ = 21945808.

The previous prime is 21945797. The next prime is 21945811. The reversal of 21945809 is 90854912.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 12054784 + 9891025 = 3472^2 + 3145^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21945809 is a prime.

It is a super-3 number, since 3×21945809³ (a number of 23 digits) contains 333 as substring.

It is a Sophie Germain prime.

Together with 21945811, it forms a pair of twin primes.

It is a Chen prime.

It is a Curzon number.

It is not a weakly prime, because it can be changed into another prime (21945829) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10972904 + 10972905.

It is an arithmetic number, because the mean of its divisors is an integer number (10972905).

Almost surely, 2^21945809 is an apocalyptic number.

It is an amenable number.

21945809 is a deficient number, since it is larger than the sum of its proper divisors (1).

21945809 is an equidigital number, since it uses as much as digits as its factorization.

21945809 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 25920, while the sum is 38.

The square root of 21945809 is about 4684.6354180448. The cubic root of 21945809 is about 279.9736752462.

The spelling of 21945809 in words is "twenty-one million, nine hundred forty-five thousand, eight hundred nine".