1999981 has 4 divisors (see below), whose sum is σ = 2027452. Its totient is φ = 1972512.

The previous prime is 1999979. The next prime is 1999993. The reversal of 1999981 is 1899991.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 1899991 = 139 ⋅13669.

It can be written as a sum of positive squares in 2 ways, for example, as 1285956 + 714025 = 1134^2 + 845^2 .

It is a cyclic number.

It is not a de Polignac number, because 1999981 - 2¹ = 1999979 is a prime.

It is a super-2 number, since 2×1999981² = 7999848000722, 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 (1999681) 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 3 ways as a sum of consecutive naturals, for example, 13626 + ... + 13771.

It is an arithmetic number, because the mean of its divisors is an integer number (506863).

2^1999981 is an apocalyptic number.

It is an amenable number.

1999981 is a deficient number, since it is larger than the sum of its proper divisors (27471).

1999981 is an equidigital number, since it uses as much as digits as its factorization.

1999981 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 27470.

The product of its digits is 52488, while the sum is 46.

The square root of 1999981 is about 1414.2068448427. The cubic root of 1999981 is about 125.9917060132.

It can be divided in two parts, 19 and 99981, that added together give a 5-th power (100000 = 10⁵).

The spelling of 1999981 in words is "one million, nine hundred ninety-nine thousand, nine hundred eighty-one".