laplace confusion....

07102017, 02:38 AM
Post: #1




laplace confusion....
Normally when we do a laplace transform, we use a function of the form f(t), a function of t, time, and the result should be a function of s, F(s), but in the example given in the user manual, they use a function of x....
e.g. If f(t) = e^(a*t), so if I try laplace(e^(a*t)), I get (e^(a*t))/x, which is wrong....should be F(s) = 1/(sa). So, I decided to use x instead since the user guide example used x....so I used: laplace(e^(a*x)) gave a result of F(x) = 1/(xa) So, am I correct in assuming that we should use x instead of t when defining f(t), and in the result from the laplace() function, we should assume again that x is really s in the results also..... ....as in the latter example, I converted f(t) to f(x) as in... f(t) = e^(a*t) > f(x) = e^(a*x) ....and likewise, in the result, we get F(x) = 1/(xa) and converting to s, we get... F(s) = 1/(sa) ....which is the correct answer. So, am I correct then, in assuming that we should always convert f(t) to f(x) before calling laplace() and convert our results from F(x) to F(s)??????????? Thx Donald 

07102017, 05:30 AM
Post: #2




RE: laplace confusion....
regarding https://wwwfourier.ujfgrenoble.fr/~par...cmd_en.pdf (page 322)
"... Or input : laplace(sin(t),t) here the variable name is t and this name is also used in the answer. Output : 1/((t)^2+1) Or input : laplace(sin(t),t,s) here the variable name is t and the variable name of the answer is s. Output: 1/((s)^2+1) " you could enter: laplace(e^(a*t),t,s) and get what you want. 

07102017, 10:01 AM
Post: #3




RE: laplace confusion....
Ah, OK, undocumented was the problem. Is laplace(f(t), t, s) to get conventional function values using appropriate variables. So, my example of laplace(e^(a*t),t,s) gives me the correct result of 1/(as), which is 1/(sa).
So, someone needs to add the 2 arguments to the documentation. Thanks Donald 

07102017, 02:56 PM
Post: #4




RE: laplace confusion....
There is an example in the online help for the laplace function.
(Enter "laplace" then press the Help button) 

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