21005917 has 4 divisors (see below), whose sum is σ = 21019516. Its totient is φ = 20992320.

The previous prime is 21005903. The next prime is 21005953. The reversal of 21005917 is 71950012.

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 6421156 + 14584761 = 2534^2 + 3819^2 .

It is a cyclic number.

It is not a de Polignac number, because 21005917 - 2¹¹ = 21003869 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21005891 and 21005900.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21005977) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 4134 + ... + 7687.

It is an arithmetic number, because the mean of its divisors is an integer number (5254879).

Almost surely, 2^21005917 is an apocalyptic number.

It is an amenable number.

21005917 is a deficient number, since it is larger than the sum of its proper divisors (13599).

21005917 is a wasteful number, since it uses less digits than its factorization.

21005917 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 13598.

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

The square root of 21005917 is about 4583.2212471143. The cubic root of 21005917 is about 275.9183271959.

Adding to 21005917 its reverse (71950012), we get a palindrome (92955929).

The spelling of 21005917 in words is "twenty-one million, five thousand, nine hundred seventeen".