21005217 has 16 divisors (see below), whose sum is σ = 32950080. Its totient is φ = 13179456.

The previous prime is 21005209. The next prime is 21005219. The reversal of 21005217 is 71250012.

It is not a de Polignac number, because 21005217 - 2³ = 21005209 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21005217.

It is not an unprimeable number, because it can be changed into a prime (21005219) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 22423 + ... + 23340.

It is an arithmetic number, because the mean of its divisors is an integer number (2059380).

Almost surely, 2^21005217 is an apocalyptic number.

21005217 is a gapful number since it is divisible by the number (27) formed by its first and last digit.

It is an amenable number.

21005217 is a deficient number, since it is larger than the sum of its proper divisors (11944863).

21005217 is a wasteful number, since it uses less digits than its factorization.

21005217 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 45789 (or 45783 counting only the distinct ones).

The product of its (nonzero) digits is 140, while the sum is 18.

The square root of 21005217 is about 4583.1448809742. The cubic root of 21005217 is about 275.9152622662.

Adding to 21005217 its reverse (71250012), we get a palindrome (92255229).

The spelling of 21005217 in words is "twenty-one million, five thousand, two hundred seventeen".