From: SourceForge.net <noreply@so...>  20090831 23:22:15

Bugs item #2846949, was opened at 20090829 20:42 Message generated for change (Comment added) made by crategus You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=104933&aid=2846949&group_id=4933 Please note that this message will contain a full copy of the comment thread, including the initial issue submission, for this request, not just the latest update. Category: Lisp Core Group: None >Status: Pending >Resolution: Works For Me Priority: 5 Private: No Submitted By: Nobody/Anonymous (nobody) Assigned to: Nobody/Anonymous (nobody) Summary: ilt(expr,s,t) cannot calculate some difficult expressions Initial Comment: Hi! I use wxmaxima 0.8.2 in Ubuntu. I have some rational function and I need to do inverse Laplace transformation. So I use ilt(), but it can't solve this. I tryed maxima 5.10.0 and 5.13.0, the result is the same (see in the file). In 5.17.1 there is a Lisp error when I'm trying to evaluate. But when I try a slightly changed function (see in the file too) it can be solved. Mathcad 14 for Windows could even solve this with the first function. The result is in the file. But when I did a Laplace transformation with the result, mathcad gave me a very difficult solvation, and using wxmaxima I got a very simple result. Both of these results weren't the primary functon. So I don't know the right answer but I really need to!  >Comment By: Dieter Kaiser (crategus) Date: 20090901 01:22 Message: Setting this bug report as pending and works for me. Dieter Kaiser  Comment By: Nobody/Anonymous (nobody) Date: 20090830 12:50 Message: I think the matter is in the complexity of calculations. I tried to find roots of the denominator, and the answer is so much difficult. I don't know which algorithm Maxima is using for ilt, but I know one, it includes search of denominator's roots. May be it's too complicated to do the transformation, I suppose, the answer is giant. I should talk with my lecturer and discuss some parameters of my transfer function. Thank you very much anyway :)  Comment By: Dieter Kaiser (crategus) Date: 20090829 21:33 Message: I have tried the examples with current Maxima 5.19post. Furthermore I have reformulated the integral a bit. It is equivalent, but looks simpler. You are right we get no solution for the following expression: (%i2) ilt(1/(s^2*(1 + a*s)*(1 + b*s)*(1 + c*s) + d*s),s,t); (%o2) 'ilt((a*b*c*s^3+((b+a)*c+a*b)*s^2+(c+b+a)*s+1) /(d*(a*b*c*s^4+((b+a)*c+a*b)*s^3+(c+b+a)*s^2+s+d)),s,t) +1/d As you have observed, we get a solution, when we omit the extra term +d*s: (%i3) ilt(1/(s^2*(1 + a*s)*(1 + b*s)*(1 + c*s)),s,t); (%o3) c^3*%e^(t/c)/(c^2+(ba)*c+a*b)b^3*%e^(t/b)/((ba)*cb^2+a*b) +a^3*%e^(t/a)/((ba)*ca*b+a^2)+tcba The Laplace transformation gives the original expression: (%i4) laplace(%,t,s); (%o4) c^3/((c^2+(ba)*c+a*b)*(s+1/c))b^3/(((ba)*cb^2+a*b)*(s+1/b)) +a^3/(((ba)*ca*b+a^2)*(s+1/a))c/sb/s a/s+1/s^2 We can see it more easy, when we factor the last result: (%i5) factor(%); (%o5) 1/(s^2*(a*s+1)*(b*s+1)*(c*s+1)) I can not see a bug. I have tried the first integral with Wolfram alpha, but get no solution too. Perhaps you can post the expected answer for the first integral. Dieter Kaiser  You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=104933&aid=2846949&group_id=4933 