21017905 has 4 divisors (see below), whose sum is σ = 25221492. Its totient is φ = 16814320.

The previous prime is 21017897. The next prime is 21017951. The reversal of 21017905 is 50971012.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 4566769 + 16451136 = 2137^2 + 4056^2 .

It is not a de Polignac number, because 21017905 - 2³ = 21017897 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21017905.

It is an unprimeable number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2101786 + ... + 2101795.

It is an arithmetic number, because the mean of its divisors is an integer number (6305373).

Almost surely, 2^21017905 is an apocalyptic number.

It is an amenable number.

21017905 is a deficient number, since it is larger than the sum of its proper divisors (4203587).

21017905 is an equidigital number, since it uses as much as digits as its factorization.

21017905 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4203586.

The product of its (nonzero) digits is 630, while the sum is 25.

The square root of 21017905 is about 4584.5288743774. The cubic root of 21017905 is about 275.9708057407.

Adding to 21017905 its reverse (50971012), we get a palindrome (71988917).

The spelling of 21017905 in words is "twenty-one million, seventeen thousand, nine hundred five".