3981

3981 is a natural number that comes after 3980 and before 3982. In number theory it is composite, with prime factorization 3 × 1327. The two prime factors are both congruent to 3 modulo 4 (3 ≡ 3 mod 4 and 1327 ≡ 3 mod 4), which makes 3981 a Blum integer, a class of semiprimes of interest in cryptography.

In various numeral systems, 3981 has the following representations: in Roman numerals it is MMMCMLXXXI; in binary it is 111110001101; in hexadecimal it is F8D; in octal it is 7615.

Its digit sum is 3+9+8+1 = 21, so the number is divisible by 3 but not by 9; its digital root is 3. Because it contains a prime factor congruent to 3 modulo 4 with odd exponent, 3981 is not expressible as a sum of two squares and is not a perfect square.

Usage: as a concrete integer, 3981 is used as an example value in exercises and demonstrations in arithmetic, base conversion, and factorization algorithms. It also serves as a simple test case in programming problems dealing with divisibility and prime factorization.