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I would like to plot the 2-D vector field:

VectorFieldPlot[{ x/Sqrt[x^2 + y^2], y/Sqrt[x^2 + y^2]}, {x, -5,  5}, {y, -5, 5} ]

but I encounter zero division at the origin. How to cast out the point {0,0}? I tried the Assumptions -> x <> 0 && y <> 0, but the string Assumptions is red and seems that it isn't available for VectorFieldPlot.

Furthermore, for another vector field, I would need to cast out the semi-line "(x,0), x in ( - infininty, 0 >".

To summarize: the question is how to restrict the (plot) domain of a vector field?

Could you help?

Thank you and wish you a nice day

marfi

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    $\begingroup$ Stackexchange etiquette: If your problem is solved, you should consider accepting the best answer. $\endgroup$
    – Philipp
    Commented Jan 21, 2015 at 11:52

2 Answers 2

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You want the RegionFunction parameter. This should be a raw Function object that returns True in the region that you do want plotted. Examples:

VectorPlot[{x/Sqrt[x^2 + y^2], y/Sqrt[x^2 + y^2]}, {x, -5, 5}, {y, -5, 5},
 RegionFunction -> Function[{x, y}, x^2 + y^2 > (0.1)^2]]

Vector plot with origin excised

VectorPlot[{x/Sqrt[x^2 + y^2], y/Sqrt[x^2 + y^2]}, {x, -5, 5}, {y, -5, 5},     
 RegionFunction ->  Function[{x, y}, (y^2 > (0.1)^2) || (x > 0.1)]]

Vector plot with negative x-axis excised

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Use VectorPlot instead.

VectorPlot[{x/Sqrt[x^2 + y^2], y/Sqrt[x^2 + y^2]}, {x, -5, 5}, {y, -5, 5}]

vectorplot

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