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Plotting Vector Field in Mathematica

  1. Oct 9, 2007 #1
    1. The problem statement, all variables and given/known data
    y'=ay-by^2-q, where a, b are positive constants, and q is an arbitrary constant. In the following, y denotes a solution of this equation that satisfies the initial condition y(0) = y_0.
    a. Choose a and b positive and q < a^2/4b. By plotting direction fields and solutions of A on suitable rectangular grids

    R = {0<t<T,c<y<d}
    in the ty plane , discover that there are numbers y1 and y2 with y1<y2 such that if
    y_0>y1 the the lim as t approaches infiniti of y(t) = y_2, while if y_0<y1 then y(t) = negative infiniti for some value of t

    2. Relevant equations

    3. The attempt at a solution

    Well I tried plotting using vectorfieldplot, it works fine for the equation w/o the constants a and b. But when i plug those in, it's error city. Do I need to manipulate this equation prior to attempting to plot it?
  2. jcsd
  3. Oct 10, 2007 #2


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    Science Advisor
    Homework Helper

    Mathematica evaluates the function being plotted and it cannot do that if it doesn't know what a and b stand for. You need to input specific values for these parameters.
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