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 Jan 17 comment Problem solving a second-order PDE The problem is that the set of equations are not a differential equation in $w$. Thus the error message from Mathematica. Jan 17 comment Problem solving a second-order PDE You are missing brackets around ps in the equation for e'. Jan 16 comment What's the pattern that will match a head anywhere in an expression How about patt[head]:= x_ /; ! FreeQ[x, _f]? Feb 16 comment Method that can be used to collect the variables of a function How about Sequence @@ f[x, y, z]? Jan 22 comment An apparently “simple” limit? @Kagaratsch: 1 is the correct result.... Dec 15 comment Remove annoying Conjugate It is actually interesting that Mathematica does not realise that the Sqrt are harmless. In fact, it can simplify the expression when removing the -2 and having the assumptions Im[es]==Im[kx]==Im[ky]==0. Sep 11 comment How to tell Mathematica that a function is dependent on other variables? Nice: +1 (and some more characters) Aug 12 comment Change variables in differential expressions I do not think the chain rule is applied correctly. If I use expr[r_]:=D[b[r],r] and then evaluate expr[1/x] I get b'[x] and not -b'[x] x^2... Jul 11 comment How can I get Mathematica to simplify $\Vert\{1,\sin(t),\cos(t)\}\Vert_2$ to $\sqrt{2}$? @term-rewritica: no, you are right $\cos^2 x + \sin^2 x$ is always 1. However $|\cos x|^2+ | \sin x|^2$ is not always equal to 1. Jun 6 comment How can I eliminate a function in a system of PDEs, Maple-style? An even easier question would be: how can one eliminate a function in a set of ordinary differential equations using Mathematica. Jun 5 comment Different determinants on 2 different computers @Öskå: Eigenvalues[{A,B}] solves the problem $\det(A - \lambda B)=0$ for $\lambda$; however in a numerically stable version unlike writing down the polynomial equation for $\lambda$ and then solving it... May 4 comment Constructing a function with Flat and OneIdentity attribute with the property that otimes[a]:>a Thank you: this is exactly the problem I was suffering. In fact, I did know that Flat affects the pattern matching but I did not know that for a pattern only the attributes at the time when it is set are important and that the attributes are then stored. May 3 comment Constructing a function with Flat and OneIdentity attribute with the property that otimes[a]:>a Thanks for your question. I figured out that my example was a bit to minimal. I changed the question such that now it is clear where the infinite recursion appears. May 3 comment Constructing a function with Flat and OneIdentity attribute with the property that otimes[a]:>a Sorry for the confusion. I tried to find a minimal example and it was too minimal (and I did not see that some old Attributes were still set). I edit the question... May 3 comment Constructing a function with Flat and OneIdentity attribute with the property that otimes[a]:>a @Rojo: otimes[anything] May 3 comment Constructing a function with Flat and OneIdentity attribute with the property that otimes[a]:>a @MarkMcClure: it also does not work with NonCommutativeMultiply. NonCommutativeMultiply[a] remains unevaluated. Apr 25 comment Incorrect results for elementary integrals when using Integrate @Searke: thank you for the suggestion, I will do so. Apr 25 comment Incorrect results for elementary integrals when using Integrate @Szabolcs: the condition which has to be met is $\operatorname{Re} x > 0$. However, I do not believe that it is because of this condition that the integral cannot be performed. Mathematica also does not produce a result for the specific case $x=1$. Apr 25 comment Incorrect results for elementary integrals when using Integrate Oh, my mistake. Mar 12 comment Does Mathematica have advanced indexing? @nikie: you are implicitly assuming that the elements of evaluatedCondition are either 0 or 1. This is not always the case (see the code for the || part). I guess the code should be modified slightly not to produce strange result in the case that the condition invokes an Or` statement.