Is there such a thing as a frequency shifting Fourier series? A Fourier cosine series looks like $$f(x) = \frac{a_0}{2}+\sum a_n cos(nx)$$ over an interval 0 < x < c. I want each frequency to shift linearly between 0 and c. Does anything like this exist as a series? For example, could I change the equation to $$f(x) = \frac{a_0}{2}+\sum a_n cos(nyx)$$ where y is a linear term. Maybe y = mx + b? If such a series exists, what is it called?
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