3661891721 has 6 divisors (see below), whose sum is σ = 3670445322. Its totient is φ = 3653358000.

The previous prime is 3661891703. The next prime is 3661891757. The reversal of 3661891721 is 1271981663.

It can be written as a sum of positive squares in only one way, i.e., 2548735225 + 1113156496 = 50485^2 + 33364^2 .

It is not a de Polignac number, because 3661891721 - 2⁶ = 3661891657 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 (3661891781) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 183221 + ... + 202221.

It is an arithmetic number, because the mean of its divisors is an integer number (611740887).

Almost surely, 2^3661891721 is an apocalyptic number.

It is an amenable number.

3661891721 is a deficient number, since it is larger than the sum of its proper divisors (8553601).

3661891721 is an frugal number, since it uses more digits than its factorization.

3661891721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 19879 (or 19440 counting only the distinct ones).

The product of its digits is 108864, while the sum is 44.

The square root of 3661891721 is about 60513.5664210927. The cubic root of 3661891721 is about 1541.3520089813.

The spelling of 3661891721 in words is "three billion, six hundred sixty-one million, eight hundred ninety-one thousand, seven hundred twenty-one".