A diagram which plots the location of sunspots on the Sun as a function of date, and is useful for tracking the 11-year solar cycle of activity. It is sometimes referred to as a Maunder diagram, after British astronomer E W Maunder of the Royal Greenwich Observatory, who started systematic solar observations in 1873, and published the original version of this chart in 1904.
In the context of fast Fourier transform algorithms, a butterfly is a portion of the computation that combines the results of smaller discrete Fourier transforms (DFTs) into a larger DFT, or vice versa (breaking a larger DFT up into subtransforms).
Most commonly, the term "butterfly" appears in the context of the Cooley-Tukey FFT algorithm, which recursively breaks down a DFT of composite size n = rm into r smaller transforms of size m where r is the "radix" of the transform. See also the Cooley-Tukey FFT article.)
Radix-2 butterfly diagram
In the case of the radix-2 Cooley-Tukey algorithm, the butterfly is simply a DFT of size 2 that takes two inputs (x0,x1) (corresponding outputs of the two sub-transforms) and gives two outputs (y0,y1) by the formula (not including twiddle factors):
y0 = x0 + x1 y1 = x0 − x1If one draws the data-flow diagram for this pair of operations, the (x0,x1) to (y0,y1) lines cross and resemble the wings of a butterfly, hence the name.
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