21355909 has 2 divisors, whose sum is σ = 21355910. Its totient is φ = 21355908.

The previous prime is 21355903. The next prime is 21355913. The reversal of 21355909 is 90955312.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 19096900 + 2259009 = 4370^2 + 1503^2 .

It is a cyclic number.

It is not a de Polignac number, because 21355909 - 2¹¹ = 21353861 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21355903) 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, 10677954 + 10677955.

It is an arithmetic number, because the mean of its divisors is an integer number (10677955).

Almost surely, 2^21355909 is an apocalyptic number.

It is an amenable number.

21355909 is a deficient number, since it is larger than the sum of its proper divisors (1).

21355909 is an equidigital number, since it uses as much as digits as its factorization.

21355909 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 12150, while the sum is 34.

The square root of 21355909 is about 4621.2453949125. The cubic root of 21355909 is about 277.4423070822.

The spelling of 21355909 in words is "twenty-one million, three hundred fifty-five thousand, nine hundred nine".