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- Phi (Φ and φ, pronounced [fī]) is a Greek letter.
- In mathematics it is used to represent the number 1.6180339887..., the key number in the Golden Ratio.
- Phi is an essential number when calculating the dimensions of a Fibonacci Spiral.
- The number 1.61803 appears on the side of the kayak Olivia Dunham sees in her dream during the Synaptic Transfer with John Scott. (Pilot)
- In Walter's Lab Notes, the image includes a grilled cheese sandwhich that has been nibbled into a shape approximating golden rectangle dimensions. (The Same Old Story)
- In Walter's Lab Notes, Walter refers to the shape of the room containing the laboratory without specifying what the shape is. (The Ghost Network)
- In the hotel where they are staying, Peter asks Walter about the numbers he recites as he is falling asleep. Walter explains that it is Fibonacci. Soon after, they realize that the numbers relate to the safety deposit boxes that are being robbed in their current investigation. Knowledge of the sequence allows them to set a trap at the next logical location in the pattern. (Safe)
- Walter uses this math again to determine the origin phenomenon of the expanding time bubbles and colliding timelines. (And Those We've Left Behind)