1981891391 has 4 divisors (see below), whose sum is σ = 1982096784. Its totient is φ = 1981686000.

The previous prime is 1981891367. The next prime is 1981891447. The reversal of 1981891391 is 1931981891.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 1931981891 = 7 ⋅275997413.

It is a cyclic number.

It is not a de Polignac number, because 1981891391 - 2²² = 1977697087 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 (1981891321) 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, 87470 + ... + 107771.

It is an arithmetic number, because the mean of its divisors is an integer number (495524196).

Almost surely, 2^1981891391 is an apocalyptic number.

1981891391 is a deficient number, since it is larger than the sum of its proper divisors (205393).

1981891391 is a wasteful number, since it uses less digits than its factorization.

1981891391 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 205392.

The product of its digits is 139968, while the sum is 50.

The square root of 1981891391 is about 44518.4387754108. The cubic root of 1981891391 is about 1256.1069456635.

The spelling of 1981891391 in words is "one billion, nine hundred eighty-one million, eight hundred ninety-one thousand, three hundred ninety-one".