1891721397 has 4 divisors (see below), whose sum is σ = 2522295200. Its totient is φ = 1261147596.

The previous prime is 1891721387. The next prime is 1891721399. The reversal of 1891721397 is 7931271981.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is not a de Polignac number, because 1891721397 - 2²⁰ = 1890672821 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1891721399) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 315286897 + ... + 315286902.

It is an arithmetic number, because the mean of its divisors is an integer number (630573800).

Almost surely, 2^1891721397 is an apocalyptic number.

It is an amenable number.

1891721397 is a deficient number, since it is larger than the sum of its proper divisors (630573803).

1891721397 is an equidigital number, since it uses as much as digits as its factorization.

1891721397 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 630573802.

The product of its digits is 190512, while the sum is 48.

The square root of 1891721397 is about 43493.9236790612. The cubic root of 1891721397 is about 1236.7608394351.

The spelling of 1891721397 in words is "one billion, eight hundred ninety-one million, seven hundred twenty-one thousand, three hundred ninety-seven".