21005817 has 32 divisors (see below), whose sum is σ = 33890304. Its totient is φ = 11318400.

The previous prime is 21005813. The next prime is 21005821. The reversal of 21005817 is 71850012.

It is an interprime number because it is at equal distance from previous prime (21005813) and next prime (21005821).

It is not a de Polignac number, because 21005817 - 2² = 21005813 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21005813) 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 in 31 ways as a sum of consecutive naturals, for example, 26298 + ... + 27084.

It is an arithmetic number, because the mean of its divisors is an integer number (1059072).

Almost surely, 2^21005817 is an apocalyptic number.

It is an amenable number.

21005817 is a deficient number, since it is larger than the sum of its proper divisors (12884487).

21005817 is a wasteful number, since it uses less digits than its factorization.

21005817 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 869.

The product of its (nonzero) digits is 560, while the sum is 24.

The square root of 21005817 is about 4583.2103377436. The cubic root of 21005817 is about 275.9178893529.

Adding to 21005817 its reverse (71850012), we get a palindrome (92855829).

The spelling of 21005817 in words is "twenty-one million, five thousand, eight hundred seventeen".