21051809 has 2 divisors, whose sum is σ = 21051810. Its totient is φ = 21051808.

The previous prime is 21051799. The next prime is 21051839. The reversal of 21051809 is 90815012.

It is a happy number.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 21022225 + 29584 = 4585^2 + 172^2 .

It is a cyclic number.

It is not a de Polignac number, because 21051809 - 2¹² = 21047713 is a prime.

It is not a weakly prime, because it can be changed into another prime (21051839) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10525904 + 10525905.

It is an arithmetic number, because the mean of its divisors is an integer number (10525905).

Almost surely, 2^21051809 is an apocalyptic number.

It is an amenable number.

21051809 is a deficient number, since it is larger than the sum of its proper divisors (1).

21051809 is an equidigital number, since it uses as much as digits as its factorization.

21051809 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 720, while the sum is 26.

The square root of 21051809 is about 4588.2250380730. The cubic root of 21051809 is about 276.1191156016.

The spelling of 21051809 in words is "twenty-one million, fifty-one thousand, eight hundred nine".