1251

1251 is an integer and is used to denote the year 1251 CE in historical contexts, placing it in the early 13th century of the Julian calendar.

In number theory, 1251 factors as 3^2 × 139; its positive divisors are 1, 3, 9, 139, 417, and 1251. As a base-10 number, its digit sum is 9, so 1251 is a Harshad number (divisible by 9, with 1251 ÷ 9 = 139). In Roman numerals, 1251 is written as MCCLI.

In other numeral systems, 1251 in binary is 10011100011, and in base-3 it is 1201100_3.

Beyond pure mathematics, 1251 appears as a numeric designation in various catalogs and classifications, including astronomical numbering where the sequence of minor planets uses sequential integers such as 1251 for individual objects.