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PROJECT EULER · #0762

Amoebas in a 2D Grid

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Consider a two dimensional grid of squares. The grid has 4 rows but infinitely many columns.

An amoeba in square (x,y) can divide itself into two amoebas to occupy the squares (x+1,y) and (x+1,(y+1)mod4), provided these squares are empty.

The following diagrams show two cases of an amoeba placed in square A of each grid. When it divides, it is replaced with two amoebas, one at each of the squares marked with B:

Originally there is only one amoeba in the square (0,0). After N divisions there will be N+1 amoebas arranged in the grid. An arrangement may be reached in several different ways but it is only counted once. Let C(N) be the number of different possible arrangements after N divisions.

For example, C(2)=2, C(10)=1301, C(20)=5895236 and the last nine digits of C(100) are 125923036.

Find C(100000), enter the last nine digits as your answer.

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