As I know, convolution is defined as $f(x)*g(x) = \int_{-\infty}^{+\infty}f(\tau)g(x-\tau)d_{\tau}$, but what if we want to convolve $f(2x)$ and $g(3x)$? It should be like $f(2x)*g(3x) = \int_{-\infty}^{+\infty}f(2\tau)g(3x-\tau)d_{\tau}$ or $f(2x)*g(3x) = \int_{-\infty}^{+\infty}f(2\tau)g(3x-3\tau)d_{\tau}$ or anything else?
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$\begingroup$ Cross-posted: math.stackexchange.com/q/2010996/14578, dsp.stackexchange.com/q/35479/5874. Please do not post the same question on multiple sites. Each community should have an honest shot at answering without anybody's time being wasted. $\endgroup$ – D.W. Dec 21 '16 at 22:43
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Replace $x$ by $x - \tau$ . So option b i.e $3x-3\tau$.
