217821987839 has 4 divisors (see below), whose sum is σ = 218468344224. Its totient is φ = 217175631456.

The previous prime is 217821987821. The next prime is 217821987863. The reversal of 217821987839 is 938789128712.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 217821987839 - 2²⁸ = 217553552383 is a prime.

It is a super-2 number, since 2×217821987839² (a number of 23 digits) contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (217821987809) 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, 323177687 + ... + 323178360.

It is an arithmetic number, because the mean of its divisors is an integer number (54617086056).

Almost surely, 2^217821987839 is an apocalyptic number.

217821987839 is a deficient number, since it is larger than the sum of its proper divisors (646356385).

217821987839 is an equidigital number, since it uses as much as digits as its factorization.

217821987839 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 646356384.

The product of its digits is 24385536, while the sum is 65.

The spelling of 217821987839 in words is "two hundred seventeen billion, eight hundred twenty-one million, nine hundred eighty-seven thousand, eight hundred thirty-nine".