155 (number)

155 (one hundred fifty-five or one hundred and fifty-five) is the natural number following 154 and preceding 156. It is a composite number.

If you add up all the primes between the least and greatest prime factors of 155, that is, 5 and 31, the result is 155. OEIS|id=A055233

It follows from the Sylow theorems that every group of order 155 has a normal subgroup of order 31 (hence is not a simple group, and is in fact solvable, thus verifying the odd order theorem), but since $5\; mid\; (31\; -\; 1)$ such a group is not necessarily a direct product of cyclic groups. However, it is a semidirect product.

There are 155 primitive permutations of degree 81. OEIS2C|id=A000019

155 is also:
* The year AD 155 or 155 BC.
* The maximum possible score in a single break in snooker, with a free ball at the start of the break (147 is the highest possible without a free ball).
* The designation of some spur routes of Interstate 55.
* Alfa Romeo 155, a model of car
* A song by the band +44.