209708127721 has 9 divisors (see below), whose sum is σ = 217202552983. Its totient is φ = 202463990680.

The previous prime is 209708127697. The next prime is 209708127787. The reversal of 209708127721 is 127721807902.

The square root of 209708127721 is 457939.

It is a perfect power (a square), and thus also a powerful number.

It can be written as a sum of positive squares in only one way, i.e., 109965855321 + 99742272400 = 331611^2 + 315820^2 .

It is not a de Polignac number, because 209708127721 - 2¹⁷ = 209707996649 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (209708127121) by changing a digit.

It is a polite number, since it can be written in 8 ways as a sum of consecutive naturals, for example, 13272336 + ... + 13288126.

Almost surely, 2^209708127721 is an apocalyptic number.

209708127721 is the 457939-th square number.

209708127721 is the 228970-th centered octagonal number.

It is an amenable number.

209708127721 is a deficient number, since it is larger than the sum of its proper divisors (7494425262).

209708127721 is an frugal number, since it uses more digits than its factorization.

209708127721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 31640 (or 15820 counting only the distinct ones).

The product of its (nonzero) digits is 197568, while the sum is 46.

The spelling of 209708127721 in words is "two hundred nine billion, seven hundred eight million, one hundred twenty-seven thousand, seven hundred twenty-one".