2155666721 has 8 divisors (see below), whose sum is σ = 2365289640. Its totient is φ = 1952301312.

The previous prime is 2155666699. The next prime is 2155666753. The reversal of 2155666721 is 1276665512.

It can be written as a sum of positive squares in 4 ways, for example, as 808776721 + 1346890000 = 28439^2 + 36700^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is a de Polignac number, because none of the positive numbers 2^k-2155666721 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (2155666021) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 1563656 + ... + 1565033.

It is an arithmetic number, because the mean of its divisors is an integer number (295661205).

Almost surely, 2^2155666721 is an apocalyptic number.

It is an amenable number.

2155666721 is a deficient number, since it is larger than the sum of its proper divisors (209622919).

2155666721 is a wasteful number, since it uses less digits than its factorization.

2155666721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3128755.

The product of its digits is 151200, while the sum is 41.

The square root of 2155666721 is about 46429.1580905792. The cubic root of 2155666721 is about 1291.7958124875.

The spelling of 2155666721 in words is "two billion, one hundred fifty-five million, six hundred sixty-six thousand, seven hundred twenty-one".