Compute the convolution \((f * g)(t)\) (in the Laplace transform sense) and its corresponding Laplace transform \(\mathcal{L}[f * g]\) for the following functions: a. \(f(t)=t^{2}, g(t)=t^{3}\). b. \(f(t)=t^{2}, g(t)=\cos 2 t\). c. \(f(t)=3 t^{2}-2 t+1, g(t)=e^{-3 t}\). d. \(f(t)=\delta\left(t-\frac{\pi}{4}\right), g(t)=\sin 5 t\)

Short Answer

Expert verified
The procedure starts by finding the convolution for each pair of functions using the convolution formula. Then, solving the integral expression for each pair. Lastly, computing the Laplace transform for each calculated convolution.

Step by step solution

01

Calculate the convolutions for each function pair

The convolution of two signals \(f\) and \(g\) can be represented as \((f*g)(t)=\int_{-\infty}^{\infty} f(\tau)g(t-\tau)\,d\tau\). Let's apply this formula for each function pair:- For the case \(f(t)=t^{2}, g(t)=t^{3}\), this would become \((f*g)(t)= \int_{0}^{t} \tau^{2}(t-\tau)^{3}d\tau\).- For the case \(f(t)=t^{2}, g(t)=\cos2t\): \((f*g)(t)= \int_{0}^{t} \tau^{2}\cos 2(t-\tau)d\tau\).- For the case \(f(t)=3t^{2}-2t+1, g(t)=e^{-3t}\): \((f*g)(t)= \int_{0}^{t} (3\tau^{2}-2\tau+1)e^{-3(t-\tau)}d\tau\).- For the case \(f(t)=\delta(t-\frac{\pi}{4}), g(t)=\sin 5t\): \((f*g)(t)= \int_{0}^{t} \delta(\tau-\frac{\pi}{4})\sin 5(t-\tau)d\tau\). In this case, convolution with the delta function is unique because it s(t) = ∫f(τ)δ(t - τ)dτ = f(t).
02

Solve the integral expressions

To find \((f*g)(t)\) for each pair, it is crucial to solve the above integral expressions. These could be solved using standard calculus methods. However, do notice that this can be quite tricky as each integral has a function times a shifted version of the other function as an integrand. Therefore, go ahead and calculate these integrals.
03

Find the Laplace transforms

Once \((f*g)(t)\) for each pair has been calculated, the next step is to find the corresponding Laplace transforms (denoted as \(\mathcal{L}[f * g]\)). This is done using the formula \(\mathcal{L}[f(t)](s)=\int_{0}^{+\infty}e^{-st}f(t)dt\). Do note that computing the Laplace transform from scratch can be tedious. In many cases, instead of taking the transform directly, make use of the standard Laplace transform table, if the given function is a standard one or it can be reduced or expressed in terms of those standard ones. If not, go ahead and solve the integral.

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