217921 has 6 divisors (see below), whose sum is σ = 239666. Its totient is φ = 198000.

The previous prime is 217909. The next prime is 217933. The reversal of 217921 is 129712.

217921 = T₄₆ + T₄₇ + ... + T₁₁₁.

It is an interprime number because it is at equal distance from previous prime (217909) and next prime (217933).

It can be written as a sum of positive squares in only one way, i.e., 148225 + 69696 = 385^2 + 264^2 .

It is not a de Polignac number, because 217921 - 2⁷ = 217793 is a prime.

It is a Duffinian number.

217921 is a lucky number.

It is a junction number, because it is equal to n+sod(n) for n = 217892 and 217901.

It is not an unprimeable number, because it can be changed into a prime (217901) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 780 + ... + 1021.

2²¹⁷⁹²¹ is an apocalyptic number.

It is an amenable number.

217921 is a deficient number, since it is larger than the sum of its proper divisors (21745).

217921 is a wasteful number, since it uses less digits than its factorization.

217921 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1823 (or 1812 counting only the distinct ones).

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

The square root of 217921 is about 466.8200938263. The cubic root of 217921 is about 60.1773456625.

Subtracting from 217921 its reverse (129712), we obtain a square (88209 = 297²).

The spelling of 217921 in words is "two hundred seventeen thousand, nine hundred twenty-one".