1105

1105 is a natural number that comes after 1104 and before 1106. In number theory it is notable for being the product of three distinct primes: 5, 13, and 17, which makes it a sphenic number.

More significantly, 1105 is a Carmichael number, the second such number known after 561. By Korselt's criterion, a composite number is Carmichael if it is square-free and p−1 divides n−1 for every prime p dividing n; for 1105, n−1 equals 1104, which is divisible by 4, 12, and 16 corresponding to primes 5, 13, and 17.

In common numeral systems, 1105 is written as 0x451 in hexadecimal and as 10001010001 in binary. In Roman numerals it is MCV.

As a calendar year, 1105 can refer to 1105 CE in the Middle Ages or 1105 BCE in ancient times. The two dates are used in historical timelines and scholarly work to pinpoint different periods across world history.

In mathematical literature, 1105 is cited as an example of a Carmichael number and a sphenic number, illustrating how a number can be composite yet satisfy certain Fermat-type conditions for many bases.