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Suppose we have the following Piecewise function:

gg[x_?NumericQ] := 
 Piecewise[{{x^4 - 6 x^3 + 5, x <= 1}, {8 x^2 - x^7, x > 1}}]

We can find the root of the function easily by:

FindRoot[gg[x], {x, 10}]
(* {x -> 1.51572} *)

The problem emerges when we make for example the second function complex:

gcom[x_?NumericQ] := 
 Piecewise[{{x^4 - 6 x^3 + 5, x <= 1}, {8 x^2 - x^7 I, x > 1}}]

In this case FindRoot does not work:

FindRoot[gcom[x], {x, 10}]

LessEqual::nord: Invalid comparison with 1.24121 -0.147854 I attempted.

Greater::nord: Invalid comparison with 1.24121 -0.147854 I attempted.

LessEqual::nord: Invalid comparison with 1.24121 -0.147854 I attempted.

Greater::nord: Invalid comparison with 1.24121 -0.147854 I attempted.

FindRoot::nlnum: The function value {\[Piecewise]   -3.813+2.96586 I    1.24121 -0.147854 I<=1
8.6317 -6.15431 I   1.24121 -0.147854 I>1
0   True

} is not a list of numbers with dimensions {1} at {x} = {1.24121 -0.147854 I}.

Note that outside the Piecewise function Mathematica can find the complex root:

FindRoot[8 x^2 - x^7 I, {x, 10}]

(* {x -> 1.44153 - 0.468382 I} *)

So my question is how can I find the complex roots of a complex-valued function in a Piecewise?

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  • $\begingroup$ The solution lies on a jump discontinuity of the function, right? How do you expect FindRoot to find that? Moreover, you equation is overdetermined: two real quations for one real parameter. $\endgroup$ Jun 14, 2018 at 16:17
  • $\begingroup$ @Schumacher: I have a piecewise function with two functions for x greater and smaller than 1. It is my question: FindRoot can find the complex root of the function, but not inside a piecewise. $\endgroup$
    – Kheeyal
    Jun 14, 2018 at 16:23
  • $\begingroup$ The problem is that your solution is complex, and your function gcom doesn't work with complex input, e.g., gcom[1+I] doesn't work. You need to change the conditions in your piecewise so that they work with complex input. $\endgroup$
    – Carl Woll
    Jun 14, 2018 at 16:24
  • $\begingroup$ I don't think that Piecewise has meaning for a complex function. FindRoot must enter a complex number for your argument. How will Piecewise deal with this. Perhaps you need to define branch cuts. $\endgroup$
    – Hugh
    Jun 14, 2018 at 16:25
  • $\begingroup$ Maybe you are looking for gcom[x_?NumericQ] := Piecewise[{{x^4 - 6 x^3 + 5, Re[x] <= 1}, {8 x^2 - x^7 I, Re[x] > 1}}]; FindRoot[gcom[x], {x, 10}]? $\endgroup$ Jun 14, 2018 at 16:26

1 Answer 1

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As I said in the comments, the solution to your FindRoot is a complex number. On the other hand, your function gcom only works when the input is a real number. This is what the error message is telling you. One possibility is to include a real check in your conditions (this is just one possibility). For example:

gcom[x_?NumericQ] := Piecewise[
    {{x^4 - 6 x^3 + 5, Element[x, Reals] && x <= 1}},
    8 x^2 - x^7 I (* otherwise *)
]

(another possibility as suggested by @Henrik in the comments is Re[x] <= 1)

Then:

FindRoot[gcom[x], {x,10}]

{x -> 1.44153 - 0.468382 I}

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  • $\begingroup$ @Woll: This works :). Thanks a lot. $\endgroup$
    – Kheeyal
    Jun 14, 2018 at 16:34

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