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Colouring a Strip

Published on Sunday, 19th May 2019, 01:00 am; Solved by 198

Problem 670

A certain type of tile comes in three different sizes - 1×1, 1×2, and 1×3 - and in four different colours: blue, green, red and yellow. There is an unlimited number of tiles available in each combination of size and colour.

These are used to tile a $2\times n$ rectangle, where $n$ is a positive integer, subject to the following conditions:

  • The rectangle 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 $2\times 12$ rectangle:

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 $F(n)$ be the number of ways the $2\times n$ rectangle can be tiled subject to these rules. Where reflecting horizontally or vertically would give a different tiling, these tilings are to be counted separately.

For example, $F(2) = 120$, $F(5) = 45876$, and $F(100)\equiv 53275818 \pmod{1\,000\,004\,321}$.

Find $F(10^{16}) \bmod 1\,000\,004\,321$.