217921905 has 12 divisors (see below), whose sum is σ = 377731380. Its totient is φ = 116224992.

The previous prime is 217921903. The next prime is 217921919. The reversal of 217921905 is 509129712.

It can be written as a sum of positive squares in 2 ways, for example, as 123676641 + 94245264 = 11121^2 + 9708^2 .

It is not a de Polignac number, because 217921905 - 2¹ = 217921903 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 217921905.

It is not an unprimeable number, because it can be changed into a prime (217921901) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 2421310 + ... + 2421399.

It is an arithmetic number, because the mean of its divisors is an integer number (31477615).

Almost surely, 2^217921905 is an apocalyptic number.

It is an amenable number.

217921905 is a deficient number, since it is larger than the sum of its proper divisors (159809475).

217921905 is a wasteful number, since it uses less digits than its factorization.

217921905 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4842720 (or 4842717 counting only the distinct ones).

The product of its (nonzero) digits is 11340, while the sum is 36.

The square root of 217921905 is about 14762.1781929362. The cubic root of 217921905 is about 601.7742896551.

The spelling of 217921905 in words is "two hundred seventeen million, nine hundred twenty-one thousand, nine hundred five".