2121615721 has 3 divisors (see below), whose sum is σ = 2121661783. Its totient is φ = 2121569660.

The previous prime is 2121615709. The next prime is 2121615757. The reversal of 2121615721 is 1275161212.

The square root of 2121615721 is 46061.

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

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It can be written as a sum of positive squares in only one way, i.e., 137804121 + 1983811600 = 11739^2 + 44540^2 .

It is not a de Polignac number, because 2121615721 - 2⁵ = 2121615689 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 2121615721.

It is not an unprimeable number, because it can be changed into a prime (2121610721) by changing a digit.

It is a polite number, since it can be written in 2 ways as a sum of consecutive naturals, for example, 23031 + ... + 69091.

Almost surely, 2^2121615721 is an apocalyptic number.

2121615721 is the 46061-st square number.

2121615721 is the 23031-st centered octagonal number.

It is an amenable number.

2121615721 is a deficient number, since it is larger than the sum of its proper divisors (46062).

2121615721 is an frugal number, since it uses more digits than its factorization.

2121615721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 92122 (or 46061 counting only the distinct ones).

The product of its digits is 1680, while the sum is 28.

The cubic root of 2121615721 is about 1284.9579280386.

Adding to 2121615721 its reverse (1275161212), we get a palindrome (3396776933).

The spelling of 2121615721 in words is "two billion, one hundred twenty-one million, six hundred fifteen thousand, seven hundred twenty-one".