The odor of raisins What would be the use, for instance, in giving the plan of the room that was really my room, in describing the little room at the end of the garret, in saying that from the window, across the indentations of the roofs, one could see the hill. I alone, in my memories of another century, can open the deep cupboard that still retains for me alone that unique odor, the odor of raisins drying on a wicker tray. The odor of raisins! It is an odor that is beyond description, one that it takes a lot of imagination to smell. But I've already said too much. If I said more, the reader, back in his own room, would not open that unique wardrobe, with its unique smell, which is the signature of intimacy. Gaston Bachelard, The Poetics of Space smellmemory
Wang tiles Wang tiles (Hao Wang, 1961) are a class of formal systems. They are modelled visually by square tiles with a color on each side. A set of such tiles is selected, and copies of the tiles are arranged side by side with matching colors, without rotating or reflecting them. The basic question about a set of Wang tiles is whether it can tile the plane or not, i.e., whether an entire infinite plane can be filled this way. The next question is whether this can be done in a periodic pattern. In 1966, Wang's student Robert Berger solved the problem in the negative. He proved that no algorithm for the problem can exist, by showing how to translate any Turing machine into a set of Wang tiles that tiles the plane if and only if the Turing machine does not halt. The undecidability of the halting problem then implies the undecidability of Wang's tiling problem. Wikipedia en.wikipedia.org Truchet TilesThe Tiling Patterns of Sebastien Truchet and the Topology of Structural Hierarchy mathalgorithms