21615809 has 2 divisors, whose sum is σ = 21615810. Its totient is φ = 21615808.

The previous prime is 21615767. The next prime is 21615851. The reversal of 21615809 is 90851612.

It is a balanced prime because it is at equal distance from previous prime (21615767) and next prime (21615851).

It can be written as a sum of positive squares in only one way, i.e., 20502784 + 1113025 = 4528^2 + 1055^2 .

It is a cyclic number.

It is not a de Polignac number, because 21615809 - 2⁸ = 21615553 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 (21615409) 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 as a sum of consecutive naturals, namely, 10807904 + 10807905.

It is an arithmetic number, because the mean of its divisors is an integer number (10807905).

Almost surely, 2^21615809 is an apocalyptic number.

It is an amenable number.

21615809 is a deficient number, since it is larger than the sum of its proper divisors (1).

21615809 is an equidigital number, since it uses as much as digits as its factorization.

21615809 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 4320, while the sum is 32.

The square root of 21615809 is about 4649.2804819671. The cubic root of 21615809 is about 278.5632568531.

The spelling of 21615809 in words is "twenty-one million, six hundred fifteen thousand, eight hundred nine".