21809009 has 2 divisors, whose sum is σ = 21809010. Its totient is φ = 21809008.

The previous prime is 21809003. The next prime is 21809057. The reversal of 21809009 is 90090812.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 21418384 + 390625 = 4628^2 + 625^2 .

It is a cyclic number.

It is not a de Polignac number, because 21809009 - 2⁴ = 21808993 is a prime.

It is a Sophie Germain prime.

It is a Curzon number.

It is not a weakly prime, because it can be changed into another prime (21809003) 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 as a sum of consecutive naturals, namely, 10904504 + 10904505.

It is an arithmetic number, because the mean of its divisors is an integer number (10904505).

Almost surely, 2^21809009 is an apocalyptic number.

It is an amenable number.

21809009 is a deficient number, since it is larger than the sum of its proper divisors (1).

21809009 is an equidigital number, since it uses as much as digits as its factorization.

21809009 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 1296, while the sum is 29.

The square root of 21809009 is about 4670.0116702210. The cubic root of 21809009 is about 279.3907202374.

The spelling of 21809009 in words is "twenty-one million, eight hundred nine thousand, nine".