Colouring a Loop

Published on Sunday, 19th May 2019, 01:00 am; Solved by 85

Problem 671

A certain type of flexible tile comes in three different sizes - 1×1, 1×2, and 1×3 - and in $k$ different colours. There is an unlimited number of tiles available in each combination of size and colour.

These are used to tile a closed loop of width $2$ and length (circumference) $n$, where $n$ is a positive integer, subject to the following conditions:

  • The loop must be fully covered by non-overlapping tiles.
  • It is not permitted for four tiles to have their corners meeting at a single point.
  • Adjacent tiles must be of different colours.

For example, the following is an acceptable tiling of a $2\times 23$ loop with $k=4$ (blue, green, red and yellow):

Acceptable colouring

but the following is not an acceptable tiling, because it violates the "no four corners meeting at a point" rule:

Unacceptable colouring

Let $F_k(n)$ be the number of ways the $2\times n$ loop can be tiled subject to these rules when $k$ colours are available. (Not all $k$ colours have to be used.) Where reflecting horizontally or vertically would give a different tiling, these tilings are to be counted separately.

For example, $F_4(3) = 104$, $F_5(7) = 3327300$, and $F_6(101)\equiv 75309980 \pmod{1\,000\,004\,321}$.

Find $F_{10}(10\,004\,003\,002\,001) \bmod 1\,000\,004\,321$.