Question

Given 5 colors to choose from, how many ways can we color the four unit squares of a $2\times 2$ board, given that two colorings are considered the same if one is a rotation of the other? (Note that we can use the same color for more than one square.)

Answer 1

165

Explanation:

we solve this by sum over the possible symmetry groups of the colorings.

1.If the board has 90 degree rotational symmetry, then all four squares are the same color. There are 5 such boards.

2. If the board has 180 degree rotational symmetry (but not 90 degree), then it has to be a 2-color checkerboard pattern. There are ⁵c₂=10 such boards.

All the remaining boards have no rotational symmetry. We can count these boards by taking all possible colorings, subtracting the ones counted above, and dividing by four (since each of these boards corresponds to four colorings). The result is (54−2×10−5)÷4=600÷4=150.

so, total number of boards= 5+10+150= 165.