1155665721 has 16 divisors (see below), whose sum is σ = 1801969664. Its totient is φ = 645022224.

The previous prime is 1155665717. The next prime is 1155665759. The reversal of 1155665721 is 1275665511.

It is a happy number.

It is not a de Polignac number, because 1155665721 - 2² = 1155665717 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is a self number, because there is not a number n which added to its sum of digits gives 1155665721.

It is not an unprimeable number, because it can be changed into a prime (1155665711) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 639001 + ... + 640806.

It is an arithmetic number, because the mean of its divisors is an integer number (112623104).

Almost surely, 2^1155665721 is an apocalyptic number.

It is an amenable number.

1155665721 is a deficient number, since it is larger than the sum of its proper divisors (646303943).

1155665721 is a wasteful number, since it uses less digits than its factorization.

1155665721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1279860.

The product of its digits is 63000, while the sum is 39.

The square root of 1155665721 is about 33995.0837769228. The cubic root of 1155665721 is about 1049.4072908845.

It can be divided in two parts, 11556 and 65721, that added together give a palindrome (77277).

The spelling of 1155665721 in words is "one billion, one hundred fifty-five million, six hundred sixty-five thousand, seven hundred twenty-one".