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BaseRepresentation
bin11110111101
32201101
4132331
530411
613101
75530
oct3675
92641
101981
111541
121191
13b95
14a17
158c1
hex7bd

1981 has 4 divisors (see below), whose sum is σ = 2272. Its totient is φ = 1692.

The previous prime is 1979. The next prime is 1987. The reversal of 1981 is 1891.

It can be divided in two parts, 19 and 81, that added together give a square (100 = 102).

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: 1891 = 3161.

It is a cyclic number.

It is not a de Polignac number, because 1981 - 21 = 1979 is a prime.

It is a super-2 number, since 2×19812 = 7848722, which contains 22 as substring.

It is the 45-th Hogben number.

It is a Duffinian number.

It is a plaindrome in base 16.

It is a nialpdrome in base 7 and base 13.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1987) by changing a digit.

It is a nontrivial repunit in base 44.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 135 + ... + 148.

It is an arithmetic number, because the mean of its divisors is an integer number (568).

It is an amenable number.

1981 is a deficient number, since it is larger than the sum of its proper divisors (291).

1981 is an equidigital number, since it uses as much as digits as its factorization.

1981 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 290.

The product of its digits is 72, while the sum is 19.

The square root of 1981 is about 44.5084261685. The cubic root of 1981 is about 12.5591859859.

The spelling of 1981 in words is "one thousand, nine hundred eighty-one".

Divisors: 1 7 283 1981