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when I type Integrate[Sign[x], x] to Mathematica, the answer is ∫Sign[x] dx so nothing new. When I type the same to Wolfram Alpha, I get a solved integral: http://www.wolframalpha.com/input/?i=Integrate%5BSign%5Bx%5D%2C+x%5D

Someone Any idea what to do otherwise? Would be really thankful for help!

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  • $\begingroup$ Try Integrate[Sign[x], {x, a, b}] $\endgroup$ Commented Dec 15, 2015 at 19:39
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    $\begingroup$ Or Integrate[Sign[x], x, Assumptions -> Element[x, Reals]] $\endgroup$ Commented Dec 15, 2015 at 19:40
  • $\begingroup$ Hey Belisarius, thanks a lot for your comment! But both of it, dont return a proper integral. Integrate[Sign[x], {x, a, b}] returns ConditionalExpression[-Abs[a] + Abs[b], Re[a] < b && a == Re[a]] and Integrate[Sign[x], x, Assumptions -> Element[x, Reals]] returns a small table: -x x<=0 x True any other ideas? $\endgroup$
    – Phawi
    Commented Dec 15, 2015 at 19:46
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    $\begingroup$ The result is correct and equivalent to the alpha result, though in a somewhat different form. ( Mathematica omits the "plus a constant" for the indefinite form, is that the issue? ) $\endgroup$
    – george2079
    Commented Dec 15, 2015 at 20:10
  • $\begingroup$ Hi George, yes thats the problem. Do you know, how I can change the indefinite form , in the how you call it "plus a constant form"? PS: Sorry for my english, I try my best $\endgroup$
    – Phawi
    Commented Dec 15, 2015 at 20:13

1 Answer 1

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As belisarius said in the comments, you need to assume x is a real variable:

res = Integrate[Sign[x], x, Assumptions -> Element[x, Reals]]
Piecewise[{{-x, x <= 0}}, x]

To get the result Alpha gets, we need to actually make use of an internal function that expresses piecewise function in terms of UnitStep.

Simplify[Simplify`PWToUnitStep[res] /. UnitStep[f_] :> (Sign[f] + 1)/2]
x Sign[x]

I think the real take away from your question though should really be the first part of this answer, i.e. you need to assume x is real to integrate this function.

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  • $\begingroup$ ok after playing around with your solution for more than an hour I have still problems with it, but I will continue trying and Reading… Hopefully I will get a way to understand it. Thanks a lot for your support! =) $\endgroup$
    – Phawi
    Commented Dec 15, 2015 at 23:01
  • $\begingroup$ @Phawi What seems to be the issue? Does the code not work if you copy, paste, and run it? $\endgroup$
    – Greg Hurst
    Commented Dec 16, 2015 at 0:08
  • $\begingroup$ its working fine, but I have problems to abstract the solution. For Example I want to integrate Sign[Sin[2 \[Pi] x] Using your way: Simplify[SimplifyPWToUnitStep[ Integrate[Sign[Sin[2 [Pi] x]], x, Assumptions -> Element[x, Reals]]] /.` Result is x Sign[Sin[2 \[Pi] x]] So, now I try to verificative this, by using the defined integral function: Integrate[Sign[Sin[2 \[Pi] x]], {x, 0, 1.5}] returns 0.5 (correct) Now I try getting same result while using the in defined integral: 1.5* Sign[Sin[2 \[Pi]*1.5]]-0* Sign[Sin[2 \[Pi]*0]] returns 1.5, what is wrong $\endgroup$
    – Phawi
    Commented Dec 16, 2015 at 14:03
  • $\begingroup$ Look at the plot here: res = Integrate[Sign[Sin[2 \[Pi] x]], x, Assumptions -> x \[Element] Reals]; Plot[res, {x, 0, 3/2}] There are jump discontinuities, and the Fundamental Theorem of Calculus requires the antiderivative to be continuous. This means you need to add a piecewise constant to your function to make it continuous before you can use FToC. Here's a great link explaining what I'm talking about. $\endgroup$
    – Greg Hurst
    Commented Dec 16, 2015 at 15:47

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