RE: RE: How can I control FindMinimum's behavior?

*To*: mathgroup at smc.vnet.net*Subject*: [mg19137] RE: RE: [mg19037] How can I control FindMinimum's behavior?*From*: Shiraz Markarian <shmarkar at chem.nwu.edu>*Date*: Thu, 5 Aug 1999 23:58:40 -0400*Organization*: Northwestern University*Sender*: owner-wri-mathgroup at wolfram.com

Hi Ted and everyone, Let me first thank you for your prompt reply. I have been following the discussion group for sometime and I really appreciate your and others in this group's willingness to help so many people with their questions. I have slowly begun to realize that the QuasiNewton method requires analytic derivatives. The problem I sent the group was perhaps an oversimplification of what I have on my hands. In the real case, the target function is the product of a known matrix A and a trial matrix X (w = A.X where A is a 73x3 matrix and X is a 3x3 matrix, initially a unity matrix). I then pick out all the negative values from the w matrix and assign a penalty function, 50*Sum(w(i,j)^2) for all negative w(i,j). It is this penalty function that I want to minimize with respect to the matrix elements of X. I wonder if GlobalOptimization is more suited for this problem? Are there other algorithms that I should/could use? I looked in MathSource and found some annealing/genetic algorithms. You might wonder why I am insisting on QuasiNewton. There are two reasons. One is that the original reference uses the BFGS algorithm and secondly, I find that when I set the Method->Automatic, the elements of X are changed one at a time (stiff equation?) and minimization inevitably returns the input values back. Apparently the BFGS algorithm worked in the original reference for an identical problem. Is there any way to input a gradient for the above problem so as to get the QuasiNewton method to work? I am using Mathematica for Students 3.0.1, if that is relevant. Thanks for your help, rajdeep kalgutkar > > Rajdeep Kalgutkar wrote: > --------------------------- > I have been using FindMinimum for a multidimensional minimization problem. I > find that when Method->Automatic is used, my input parameters values are > passed on to my target function immediately, but when I use > Method->QuasiNewton, FindMinimum insists on symbolic input during the first > minimization cycle. This happens even though I specify 2 input values. My > target function does not have symbolic derivatives and therefore it the > minimization procedure crashes when using the QuasiNewton method. Is there > anyway to prevent FindMinimum from insisting on symbolic input initially? > ---------------------------- > > Apparently the QuasiNewton method needs to have the symbolic derivative. In > your example the function does have a symbolic derivative. The only problem > is that Mathematica can't figure it out because of the way (func) is > defined. What you need to do is use the Gradient option to tell the kernel > what the derivative is. It's called Gradient because that's what you need > for multi-dimensional problems. Also the Quasi-Newton method only needs one > starting point. > > My solution is below. > -------------------------- > > In[1]:=func[x_]:=Module[{w}, > w=Sin[x]^2+0.2*x; > Print[w," ",x]; > w > ] > > > > In[2]:= > FindMinimum[func[x], {x,0.}, > Method->QuasiNewton, > Gradient->{2 Sin[x]Cos[x]+0.2} > ] > > > > 0. 0. > > -0.000530497 -0.2 > > -0.0100297 -0.102717 > > -0.0100337 -0.100623 > > -0.0100337 -0.100679 > > > Out[2]= > {-0.0100337,{x->-0.100679}} > > > ----------------- > Regards, > Ted Ersek >

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