Fourier series is a technique of decomposing a periodic signal into a sum of sine and cosine terms. Fourier Transform is a mathematical operation for converting a signal from time domain into its frequency domain. They are related in that they both deal with the decomposition of a signal into its constituent frequency components. However, they are different mathematical techniques used for different types of signals.

The Fourier series is used to decompose a periodic signal into a series of harmonic functions. It represents a periodic signal as a sum of sine and cosine waves of different frequencies, each with a specific amplitude and phase.

The Fourier transform, on the other hand, is used to decompose a non-periodic signal into its frequency components. It represents a non-periodic signal as a continuous spectrum of sine and cosine waves of different frequencies, each with a specific amplitude and phase.
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