1665828821 has 4 divisors (see below), whose sum is σ = 1683002412. Its totient is φ = 1648655232.

The previous prime is 1665828757. The next prime is 1665828917. The reversal of 1665828821 is 1288285661.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 1288285661 = 13 ⋅99098897.

It can be written as a sum of positive squares in 2 ways, for example, as 739296100 + 926532721 = 27190^2 + 30439^2 .

It is a cyclic number.

It is not a de Polignac number, because 1665828821 - 2⁶ = 1665828757 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1665828421) 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, 8586650 + ... + 8586843.

It is an arithmetic number, because the mean of its divisors is an integer number (420750603).

Almost surely, 2^1665828821 is an apocalyptic number.

It is an amenable number.

1665828821 is a deficient number, since it is larger than the sum of its proper divisors (17173591).

1665828821 is an equidigital number, since it uses as much as digits as its factorization.

1665828821 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 17173590.

The product of its digits is 368640, while the sum is 47.

The square root of 1665828821 is about 40814.5662845999. The cubic root of 1665828821 is about 1185.4323930194.

The spelling of 1665828821 in words is "one billion, six hundred sixty-five million, eight hundred twenty-eight thousand, eight hundred twenty-one".