21009217 has 2 divisors, whose sum is σ = 21009218. Its totient is φ = 21009216.

The previous prime is 21009211. The next prime is 21009271. The reversal of 21009217 is 71290012.

Together with next prime (21009271) it forms an Ormiston pair, because they use the same digits, order apart.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 17564481 + 3444736 = 4191^2 + 1856^2 .

It is a cyclic number.

It is not a de Polignac number, because 21009217 - 2²³ = 12620609 is a prime.

It is not a weakly prime, because it can be changed into another prime (21009211) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10504608 + 10504609.

It is an arithmetic number, because the mean of its divisors is an integer number (10504609).

Almost surely, 2^21009217 is an apocalyptic number.

It is an amenable number.

21009217 is a deficient number, since it is larger than the sum of its proper divisors (1).

21009217 is an equidigital number, since it uses as much as digits as its factorization.

21009217 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 252, while the sum is 22.

The square root of 21009217 is about 4583.5812417803. The cubic root of 21009217 is about 275.9327752329.

Adding to 21009217 its reverse (71290012), we get a palindrome (92299229).

The spelling of 21009217 in words is "twenty-one million, nine thousand, two hundred seventeen".