20191981 has 4 divisors (see below), whose sum is σ = 20282752. Its totient is φ = 20101212.

The previous prime is 20191931. The next prime is 20191987. The reversal of 20191981 is 18919102.

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, and also an emirpimes, since its reverse is a distinct semiprime: 18919102 = 2 ⋅9459551.

It is a cyclic number.

It is not a de Polignac number, because 20191981 - 2¹⁹ = 19667693 is a prime.

It is a super-2 number, since 2×20191981² = 815432193408722, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (20191987) 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, 45051 + ... + 45496.

It is an arithmetic number, because the mean of its divisors is an integer number (5070688).

Almost surely, 2^20191981 is an apocalyptic number.

It is an amenable number.

20191981 is a deficient number, since it is larger than the sum of its proper divisors (90771).

20191981 is an equidigital number, since it uses as much as digits as its factorization.

20191981 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 90770.

The product of its (nonzero) digits is 1296, while the sum is 31.

The square root of 20191981 is about 4493.5488202533. The cubic root of 20191981 is about 272.3075250525.

The spelling of 20191981 in words is "twenty million, one hundred ninety-one thousand, nine hundred eighty-one".