[MUG] Help with dsolve (Maple 6 vs Maple 7)

[MUG] Re: Help with dsolve (Maple 6 vs Maple 7)
Author: Maple User Group    Posted: Fri, 15 Nov 2002 10:49:09 -0500
>> From: Maple User Group "maple_gr" -=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=- Date: Mon, 11 Nov 2002 16:44:06 -0800 (PST) From: Robert Israel "israel" To: "maple-list" Subject: Help with dsolve (Maple 6 vs Maple 7) Your differential equation contains y as well as y(x). That's the problem. Change every y to y(x) and it should work. The error message in Maple 8 is only slightly less confusing: > S:= dsolve({diff(y(x),x)=y, y(0)=1}, y(x), numeric); S := proc(x_rkf45) ... end proc > S(1); Error, (in S) global 'y' must be assigned to a numeric value before obtaining a solution If you don't include the numeric option, the dsolve command returns an error message which is more to the point: > dsolve({diff(y(x),x) = y, y(0)=1}, y(x)); Error, (in ODEtools/info) y(x) and y cannot both appear in the given ODE. I don't know why `dsolve/numeric` doesn't check for this rather common error and return an intelligible error message. Robert Israel "israel" Department of Mathematics http://www.math.ubc.ca/~israel University of British Columbia Vancouver, BC, Canada V6T 1Z2 -=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=- Date: Mon, 11 Nov 2002 19:57:10 -0800 From: Allan Wittkopf "awittkop" Subject: Help with dsolve (Maple 6 vs Maple 7) To: "maple-list" Hello Jim, In Maple 8, the reason for this appears to be made clearer by the error message. In M8 we get: > ff(3000); Error, (in ff) global 'y' must be assigned to a numeric value before obtaining a solution so clearly y is not being recognized as the same thing as y(x), which technically it isn't. There was a significant overhaul of the dsolve/numeric suite from M6->M7, and even further changes from M7->M8, and it seems in that overhaul that y and y(x) are no longer considered synonymous. If you try the following in Maple 7 (where all that has been done is to replace y with y(x)), you get the expected results: > restart: > g:=32.2: b:=10: m:=1.4: Q:=450: Kn:=1.486: So:=0.0008: n:=0.015: > A:=y(x)*b+y(x)^2*m: T:=b+2*y(x)*m: > P:=b+2*y(x)*sqrt(1+m^2): > Ahc:=(y(x)^2*b/2)+(y(x)^3/3*m): > E:=y(x)+z+(Q/A)^2/(2*g)*(1+k): > Fr:=Q/sqrt(g*A^3/T): > ManQ:=Kn/n*A^(5/3)/P^(2/3)*sqrt(S): > Mo:=Ahc+Q^2/(g*A): > Sf:=solve(ManQ^2=Q^2,S): > ode1:= diff(y(x),x)+(So-Sf)/(1-Fr^2): > ff:=dsolve({ode1,y(0)=6},{y(x)},type=numeric): bytes used=1023732, alloc=917336, time=0.06 > ff(3000); [x = 3000., y(x) = 4.89395384081212192] Sincerely, Allan Wittkopf -=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=- Date: Tue, 12 Nov 2002 07:35:00 +0100 From: Preben Alsholm "P.K.Alsholm" To: "maple-list" Subject: Help with dsolve (Maple 6 vs Maple 7) Replace y by y(x) as has always been necessary when using dsolve without type=numeric. It was rather inconsistent that y and y(x) both could appear when using type=numeric, but not when not. Preben Alsholm Department of Mathematics Technical University of Denmark -=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=- Date: Tue, 12 Nov 2002 09:31:22 +0100 From: Koch-Beuttenmueller "Heike.Koch-Beuttenmueller" To: "maple-list" Subject: Help with dsolve (Maple 6 vs Maple 7) If I substitute y by y(x) I get your result with Maple7 and Maple8. ode1:= diff(y(x),x)+(So-Sf)/(1-Fr^2); 4/3 (10. + 3.440930106 y(x)) Sf := 20.63335863 ---------------------------------- 2 10/3 (10. y(x) + 1.400000000 y(x) ) 4/3 20.63335863 (10. + 3.440930106 y(x)) .0008 - --------------------------------------- 2 10/3 /d \ (10. y(x) + 1.400000000 y(x) ) ode1 := |-- y(x)| + ----------------------------------------------- \dx / 6288.819875 (10 + 2.8 y(x)) 1 - --------------------------- 2 3 (10 y(x) + 1.4 y(x) ) > ff:=dsolve({ode1,y(0)=6},{y(x)},type=numeric); ff := proc(rkf45_x) ... end proc > ff(3000); [x = 3000., y(x) = 4.89395314199846342] Mit freundlichen Gruessen Heike Koch-Beuttenmueller ======================================================= Dr. Heike Koch-Beuttenmueller Universitat Ulm Universitatsrechenzentrum Albert-Einstein-Allee 11 D-89081 Ulm Germany e-mail: "Heike.Koch-Beuttenmueller" Telefon: 0731 5022465 ======================================================= -=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=- Date: Tue, 12 Nov 2002 08:31:27 -0500 From: "Douglas B. Meade" "meade" To: "maple-list" Jim Shoemaker "shoe" Subject: Help with dsolve (Maple 6 vs Maple 7) Jim and MUG, The problem with your differential equation is that you use both y(x) and y. In Maple 6 and earlier, Maple was not picky about the inconsistent references to the unknown function. As of Maple 7, the user must be consistent and must use the form that explicitly shows the dependence. (There are shortcuts to this using aliases or depends.) In Maple 8, the error message from your original code is: > ff:=dsolve({ode1,y(0)=6},{y(x)},type=numeric): > ff(3000); Error, (in ff) global 'y' must be assigned to a numeric value before obtaining a solution This message is almost informative enough to diagnose the problem. When all references to y are changed to y(x), Maple 8 produces the following response to the evaluation of the numeric solution: > ff:=dsolve({ode1,y(0)=6},{y(x)},type=numeric): > ff(3000); [x = 3000., y(x) = 4.89395330563466400] This is consistent with the solution you report for Maple 6. I hope this explains the result you are seeing. Doug ----------------------------------------------------------------------- Prof. Douglas B. Meade Phone: (803) 777-6183 FAX: (803) 777-6527 Department of Mathematics URL: http://www.math.sc.edu/~meade/ USC, Columbia, SC 29208 E-mail: "mailto:meade" -=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=- From: "Dr Francis J. Wright" "F.J.Wright" To: "shoe" Subject: Help with dsolve (Maple 6 vs Maple 7) Date: Tue, 12 Nov 2002 13:32:44 -0000 What surprises me is that it worked in Maple 6! You have mixed up y and y(x); diff requires the dependent variable to be y(x) consistently. An easy fix is to include a subs as follows: ode1:= diff(y(x),x)+subs(y=y(x),(So-Sf)/(1-Fr^2)): With that change, your code works for me and gives the same result in Maple 6, 7 and 8. The Maple 7 error message is a red herring (inappropriate and misleading); Maple 8 gave a much more helpful error message. Francis

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