Sample Problems

Group theory

Let nn beads be arranged on a circle and indexed 0,1,,n10,1,\dots,n-1. Each bead is colored by an element of the finite field F7=Z/7Z\mathbb{F}_7=\mathbb{Z}/7\mathbb{Z}. Two colorings (c0,,cn1)F7n(c_0,\dots,c_{n-1})\in \mathbb{F}_7^n are considered equivalent if one can be obtained from the other by a rotation or reflection of the necklace (that is, by the usual dihedral action DnD_n on indices).

A coloring is admissible if it satisfies all three linear constraints in F7\mathbb{F}_7:

i=0n1ci0(mod 7)i=0n1ici0(mod 7)0in1i0 (mod 3)ci1(mod 7)\begin{aligned} \sum_{i=0}^{n-1} c_i &\equiv 0 &&(\mathrm{mod}\ 7) \\ \sum_{i=0}^{n-1} i\,c_i &\equiv 0 &&(\mathrm{mod}\ 7) \\ \sum_{\substack{0\le i\le n-1\\ i\equiv 0\ (\mathrm{mod}\ 3)}} c_i &\equiv 1 &&(\mathrm{mod}\ 7) \end{aligned}

For this problem, take n=30n=30. How many DnD_n-equivalence classes of admissible colorings are there for n=30n=30? Report your answer modulo 1,000,0031{,}000{,}003 as a positive integer.

Answer: 587104