21155905 has 8 divisors (see below), whose sum is σ = 26880552. Its totient is φ = 15929088.

The previous prime is 21155903. The next prime is 21155909. The reversal of 21155905 is 50955112.

It can be written as a sum of positive squares in 4 ways, for example, as 2076481 + 19079424 = 1441^2 + 4368^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 21155905 - 2¹ = 21155903 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21155903) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 124362 + ... + 124531.

It is an arithmetic number, because the mean of its divisors is an integer number (3360069).

Almost surely, 2^21155905 is an apocalyptic number.

It is an amenable number.

21155905 is a deficient number, since it is larger than the sum of its proper divisors (5724647).

21155905 is a wasteful number, since it uses less digits than its factorization.

21155905 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 248915.

The product of its (nonzero) digits is 2250, while the sum is 28.

The square root of 21155905 is about 4599.5548697673. The cubic root of 21155905 is about 276.5734811412.

The spelling of 21155905 in words is "twenty-one million, one hundred fifty-five thousand, nine hundred five".