9866 = 2 x 4933.
9866 has a representation as a sum of two squares: 9866 = 292 + 952.
9866 is the number of words of length 45 in a certain language (A005819).
9866 is the sum of the proper divisors of the number of partitions of 34 (A139055).
9866 is the number of 6 x 6 arrays of squares of integers, symmetric about both diagonal and antidiagonal with all rows summing to 26 (A156389 and A156485).
Source: On-Line Encyclopedia of Integer Sequences