MathDB

Given an

m\times n

grid with unit squares coloured either black or white, a black square in the grid is stranded if there is some square to its left in the same row that is white and there is some square above it in the same column that is white. Find a closed formula for the number of

2\times n

grids with no stranded black square. Note that

n

is any natural number and the formula must be in terms of

n

with no other variables.

Problem Statement