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Homework Help: Proving Characteristics of Projections

  1. Nov 11, 2011 #1
    Can anyone think of a trivial proof for the fact that the projection of any polynomial onto the xy-plane itself gives a polynomial? My professor and I were speculating about this, but could not discover a trivial proof for the fact. Does such a thing exist?
     
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  3. Nov 11, 2011 #2

    HallsofIvy

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    First, it would help if you would explain what you are talking about. A "polynomial" is not a geometric figure and so has no "projection". Do you mean the graph of a polynomial in x, y, and z.
     
  4. Nov 11, 2011 #3
    Yes, that's exactly what I meant.
     
  5. Nov 11, 2011 #4

    Bacle2

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    I am not sure I understand: you have , I guess, a polynomial P(x,y,z) . Then evaluate P(x,y,0). This is a polynomial in (x,y).
     
  6. Nov 11, 2011 #5
    But a projection onto the xy-plane isn't the same as letting the z-coordinate equal zero. It may even be that the polynomial doesn't intersect with the xy-plane.
     
  7. Nov 11, 2011 #6

    Bacle2

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    I'm sorry, it may be just me, but I'm not sure I understand your layout. Maybe a formal definition and/or some examples may help, if you could provide them. I understand, e.g., the projection of the unit sphere x2+y2+z=1 into the xy-plane to be the circle x2+y2=1 , but I don't understand well what you're doing.
     
  8. Nov 12, 2011 #7

    HallsofIvy

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    No, the projection of the unit sphere, [itex]x^2+ y^2+ z^2= 1[/itex] onto the xy-plane is NOT the circle, [itex]x^2+ y^2= 1[/itex]. It is the disk [itex]x^2+ y^2\le 1[/itex]. That is NOT a polynomial function, it is not even a function.

    The only interpretation I can put on this, so that it is true, is that a polynomial path in three dimensions, given by parametric equations, x= p(t), y= q(t), z= r(t), where p, q, and r are polynomials, projected to the xy-plane, which would indeed be x= p(t), y= q(t), z= 0, is a polynomial path in the xy-plane since p and q are still polynomials.
     
  9. Nov 13, 2011 #8

    Bacle2

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    Yes, my bad, H of Ivy, i was obviously wrong, I was thinking about something else, also trying to make sense of the OP.
     
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