# Tag Info

25

Fixed (see below) Here's an approach: r1 = Exp[-x^3 - y] - 1 == z; r2 = y == z; We create ImplicitRegions: reg1 = ImplicitRegion[r1, {x, y, z}]; reg2 = ImplicitRegion[r2, {x, y, z}]; The intersection of these regions is the line you seek: reg = RegionIntersection[reg1, reg2]; And here is the length (note the inclusion of the range of values in ...

23

The plan is first get the "external" contour and then use Green's theorem to find its area. r[t_] := {-9 Sin[2 t] - 5 Sin[3 t], 9 Cos[2 t] - 5 Cos[3 t], 0} (*find the intersections*) tr = Quiet@ToRules@Reduce[{r@t1 == r@t2, 0 < t1 < t2 < 2 Pi}, {t1, t2}]; pt = {t1, t2} /. {tr} // Flatten; pts = SortBy[pt, N@# &]; pps = Partition[pts, 2]; Now ...

19

Summary: Setting GenerateConditions -> False turns off safety checks. In my opinion, when the user does that and the result is erroneous, I would not call that a bug. Now WRI could decide to improve Mathematica in this case, but it might not be such a simple matter. On the other hand, it is entirely up to the user to decide whether or not he or she is ...

17

Since another answer provides misleading results I'm motivated to explain what's wrong there and suggest a correct solution. One can guess that the error there has been produced by DiscretizeRegion or by an improper parametrization of the curve with RegionIntersection (more likely the former reason). Let's start from the begining defining the following ...

17

SphericalPlot3D is having problems where the radius goes to infinity. You can use RegionFunction to restrict the plotting region to a range where the function is well-behaved: SphericalPlot3D[ Sqrt[1/(Sin[θ]^2 Cos[2 ϕ] - Cos[θ]^2)], {θ, 0, π}, {ϕ, 0, 2 π}, MaxRecursion -> 4, PlotRange -> {-3, 3}, RegionFunction -> Function[{x, y, z, θ, ϕ, ...

16

Having experienced similar problematic issues with Mathematica I instantly thought that expanding the fraction in the integrand i.e. applying Appart could resolve the problem, and indeed it does: Integrate[ Apart[(1 - x)(1 + 2x)^6/Sqrt[1 - x^2]], {x, -1, 1}]/Pi 15 These arguments apply to this case as well Bug in mathematica analytic integration? i.e. ...

16

$Version (* "8.0 for Microsoft Windows (64-bit) (October 7, 2011)" *) Direct attack fails: Timing[Convolve[Sinc[x], Exp[-x^2], x, y]] (* Out[218]= {59.296, Convolve[Sinc[x], E^-x^2, x, y]} *) or, equivalently, Timing[Integrate[Sinc[x] Exp[-(x - y)^2], {x, -∞, ∞}] ]$\left\{49.92,\int_{-\infty }^{\infty } e^{-(x-y)^2} \text{Sinc}[x] \, dx\right\}$... 14 Update a working approach using Fourier transforms The following method is based on the idea that the Laplace operator is just a multiplication with$-k^2$in Fourier space. Therefore, I first find the Fourier transform of the$1/r$potential and then do an inverse Fourier transform of that result, multiplied by$-k^2$(which I call laplacianFT in the ... 14 The "canonical" way in Mathematica is f[x_, y_] := x^2 + y^2 g[x_, y_] := x^4 + 4 x y + 2 y^4 - 8 Maximize[{f[x, y], g[x, y] == 0}, {x, y}] If you want to make explicit usage of the Lagrange multiplier: ss = N@Solve[Grad[f[x, y] + λ g[x, y], {x, y}] == 0 && g[x, y] == 0, {x, y, λ}, Reals] gives the {x, y} coordinates of the maxs and mins. ... 13 The integrals can in fact be done exactly, but only if you make some use of the symmetries of the problem first. The circular ring geometry implies that the magnetic field will look the same in any vertical plane going through the rotation axis (which we call the z axis). Therefore, we don't need to specify three independent variables x, y and z to do the ... 13 As you notniced, the result is not correct. This is not uncommon with definite integrals, as the system needs to detect any special behaviour inbetween the integration bounds, which is hard. For this reason it's a good idea to verify such integrals numerically. We can get the correct result like this (please evaluate in a fresh kernel without ... 13 [This is not a full response, but too much detail for a comment.] The general rule is that any integral that can behave differently on a measure zero set in the space of real values of parameters is a candidate for giving a result that will not be what you want. There are other caveats as well, for example in dealing with multiple integrals. And sometimes ... 13 Yes, this is a bug in the more general function RegionMeasure. I knew there were some edge cases in the handling of inexact numberics, but I was unaware of such a simple example. I will forward this bug internally. Workarounds include using the parametric (2-argument) form of ArcLength, and using DiscretizeRegion to preprocess regions before sending them ... 13 Introduction as of 31 January 2015 This is a very interesting problem which on trying to solve it with Mathematica requires skilled handling in an interactive way ("man-machine-interaction"). What do you do when Mathematica refuses to solve an integral? How to help solving integrals which Mathematica declares (erroneously) as divergent. I have done an ... 13 There is a specific function called CountRoots, which does exactly what you want: CountRoots[poly, x] (* 12 *) You can then compare this number to the number of roots given by the length of the coefficient list: Exponent[poly, x] (* 36 *) 13 Update: a way of joining (taking the union of) the first rectangle and first circle, so that the edge form only marks the exterior of the joined form p = 100; curve = Line@ Table[{.53, 0} + {.07, Sqrt[0.5]} {Sin[-2 Pi k /p], Cos[-2 Pi k /p]}, {k, -1 + p/2}]; poly = Polygon[Join[t1 = Table[{.47, 0} + {.07, Sqrt[0.5]} {Sin[-2 Pi k /p], ... 12$Version (* Out[228]= "8.0 for Microsoft Windows (64-bit) (October 7, 2011)" *) The result provided by Mathematica is correct: Integrate[a/(a^2 + Sin[t]^2), {t, 0, 2 π}] (* Out[213]= (2 π)/(Sqrt[1 + 1/a^2] a) *) Now the same procedure as "always" which "explains" the zero result. The indefinite integral is Integrate[a/(a^2 + Sin[t]^2), t] (* ...

11

I suspect you are correct in your assessment. Since there are approximate numbers in the input, Maximize punts to NMaximize, which uses penalty methods to enforce some constraints (not sure why it needs them here for linear constraints; I need to check into that). You can get better behavior by forcing real values. NMaximize[{Re[(h*10)/(300*(100 - (l^.5 + ...

11

Use NDSolve antiD = NDSolveValue[{f'[x] == Sqrt[1 + x^3], f[0] == 0}, f, {x, 0, 10}] Example usage: Plot[antiD[x], {x, 0, 10}] Alternatively... This works because this function can be antidifferentiated (by Mathematica). antiD = FunctionInterpolation[ Evaluate @ Integrate[Sqrt[1 + x^3], {x, 0, t}, Assumptions -> 0 < t < 10], {t, 0, ...

11

$$\mathbb{P} \big(\sum_{i=1}^{m} (A_i + S_i) \le L < \sum_{i=1}^{m+1} (A_i + S_i) \big) = \frac{\mu ^m (2 \lambda +\mu )}{2^{m+1} (\lambda +\mu )^{m+1}}$$ Observing: d1 = TransformedDistribution[ a + s, {a \[Distributed] ExponentialDistribution[λ], s \[Distributed] ExponentialDistribution[μ]}] (* ...

11

Chances are it will behave similarly to Integrate[Exp[-x t^3 ], {t, 0, π/2}]. I show a numerical verification below. We'll use the dominant series term, which we can compute, for the above integral, and compare to numerical integrations of the one we cannot compute analytically. ee1[x_] := Exp[-x*t^3] ee2[x_] := Exp[-x*t^3*Cos[t]]; Let's get the candidate ...

11

I've got a theory.... Let's try to get the antiderivative: Integrate[(1 + 16*Tan[2*x - y]^2)/(1 + 4*Tan[2*x - y]^2), x, Assumptions -> y \[Element] Reals] (*returns 5 x - (5 y)/2 - ArcTan[2 Tan[2 x - y]] *) Seems legit. You can test with D[5 x - (5 y)/2 - ArcTan[2 Tan[2 x - y]], x] and plot or rearrange. This antiderivative is technically correct, ...

11

Just purely for fun: to illustrate approximation of volume integral. Manipulate[ Module[{dp, dpl, dpu, r = Range[0, 1, 1/n], f, cc, lb, la, ub, ua, ll, ul}, dp = Range[0, 1, 1/#] & /@ Range[2, 100]; dpl = Total[Pi Most[#]/(Length@# - 1)] & /@ dp; dpu = Total[Pi Rest[#]/(Length@# - 1)] & /@ dp; f = Sqrt /@ r; cc = Partition[{#, 0, ...

10

Using a symbolic functionality like Maximize with an expression involving approximate numbers is not in general a good idea, even though Maximize calls automatically NMaximize in such cases. However if we rewrite the expression to an exact form, then Maximize will run very long time returning no symbolic results. The problem one encounters here is most ...

10

Thanks to Artes for helping me sort out bugs in this answer. Solve[E^(-x^3 - y) - 1 - y == 0, y] {{y -> -1 + ProductLog[E^(1 - x^3)]}} To account for that fact that $-4 < y = z < 4$ we need to find the range of allowed $x$ values. FindRoot[-1 + ProductLog[E^(1 - x^3)] == 4, {x, 1}] {x -> -1.77681} FindRoot[-1 + ProductLog[E^(1 - ...

10

Brute force approach: g[y0_] := x /. FindRoot[ Exp[-(x^3 + y)] - 1 == y /. y -> y0 , {x, -4, 4}] line = Table[{g[y], y, y}, {y, -1, 4, .0001}]; Graphics3D[Line@line] Total[Norm@(Subtract @@ #) & /@ Partition[line, 2, 1]] 10.9513

10

Solve also works Solve[y == 30 x^2 (1 - x)^2 && 0 < x < 1, x, Reals] {{x -> ConditionalExpression[Root[-y + 30 #1^2 - 60 #1^3 + 30 #1^4 &, 2], 0 < y < 15/8]}, {x -> ConditionalExpression[Root[-y + 30 #1^2 - 60 #1^3 + 30 #1^4 &, 3], 0 < y < 15/8]}} Use ToRadicals to get it in a nice looking form.

10

I have no idea why Mathematica complains about the convergence. When you expand the integrand, then integrate each of the terms and add, you will find the exact result: E^(-((201 x1^2)/101)) x1^2 (15 - 20 x1^2 + 4 x1^4) (5913508078417951503 + 1124782662003060300000 x1^2 - 2983951574394000000000 x1^4 + 2591462040000000000000 x1^6 - 797900000000000000000 ...

10

Here is a visualization using MeshFunctions (as mentioned by Daniel in the comment): f = x y z; g = x^2 + 10 y^2 + z^2; gp = With[{r = 3}, RegionPlot3D[g < 5, {x, -r, r}, {y, -r, r}, {z, -r, r}, PlotStyle -> Orange, PlotPoints -> 40, Mesh -> None, ViewPoint -> Front, PlotTheme -> "Classic"] ]; With[{r = 3}, Manipulate[ ...

10

For what it's worth, you can monitor the progress of Integrate and NIntegrate. I'm not sure how helpful the tools below are, but I feel it is probably worth mentioning them. Integrate InternalIntegratedebugSwitch If you set InternalIntegratedebugSwitch to the magic number 10, it will print its (major) steps in searching for the answer. For instance: ...

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