You just take the index, the index plus one, and the index minus one, for every other dimension, but exclude the index of where you're at. For example if you have a 3D matrix, and you're at x=3, y=6, and z=9, you'd have all permutations of x in [2,3,4] with y in [5,6,7] with z in [8,9,10] but don't include the point itself with x=3,y=6, z=9. So that's 3*3*3-1 neighbors in 3D. For N dimensions you'd have 3 * 3 * 3 * ....(a total N times) * 3 -1 neighbors. So 2D gives 3*3-1 = 8 neighbors, 3D gives 26 neighbors, 4D gives 80 neighbors, etc.