Fibonacci sequence
geometrical progression or series
1, 2, 3, 5, 8, ...

The series may be defined better with two equal start terms 1, which is obvious in the figure. We start with a unit square and place a square with that size along. They form a rectangle with length 2, which is the side of a square added upside. The just formed rectangle measures 2x3, prescribing the next square. (Follow the red curve.) The total rectangle is 8x13. With every step, the exact shape of the rectangle deviates less from the so-called Golden Rectangle.