217540721 has 4 divisors (see below), whose sum is σ = 230337252. Its totient is φ = 204744192.

The previous prime is 217540703. The next prime is 217540777. The reversal of 217540721 is 127045712.

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 183900721 + 33640000 = 13561^2 + 5800^2 .

It is a de Polignac number, because none of the positive numbers 2^k-217540721 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (217530721) 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, 6398240 + ... + 6398273.

It is an arithmetic number, because the mean of its divisors is an integer number (57584313).

Almost surely, 2^217540721 is an apocalyptic number.

It is an amenable number.

217540721 is a deficient number, since it is larger than the sum of its proper divisors (12796531).

217540721 is a wasteful number, since it uses less digits than its factorization.

217540721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 12796530.

The product of its (nonzero) digits is 3920, while the sum is 29.

The square root of 217540721 is about 14749.2617103366. The cubic root of 217540721 is about 601.4232149480.

The spelling of 217540721 in words is "two hundred seventeen million, five hundred forty thousand, seven hundred twenty-one".