198 is the 4th term of the sequence a(n) = 10*a(n-1) - a(n-2) with a(0) = 0, a(1) = 2. This sequence has the property that for each n, if A = a(n), B = 2*a(n+1), C = 3*a(n+1) then A*B+1, A*C+1, B*C+1 are perfect squares.[6]
198 is the first number between a twin prime pair such that product of itself by its reversal (198 x 891 = 176418) is also between a twin prime pair.[3]
198 is a
Harshad number, which means it is divisible by the sum of its digits.[7]
198 is both the average of a twin prime pair (197, 199), and the sum of 2 successive primes (97, 101).[8][9]
198 is a coordination number for
hexagonal lattices, which means it is the difference between two consecutive
centered hexagonal numbers.[10] In this case, 198 is the difference between 3367 and 3169 (34th and 33rd centered hexagonal numbers), so the hexagonal spatial arrangement of 3367 has 198 dots in the border.
198 is also the 8th coordination number for cubic lattices, making it the 6th number to be part of the
coordination sequences for both cubic and hexagonal lattices.[11]