# 352 - Topic 03 - Qualitative Solutions

## 2244 days ago by Professor352

Solving Separable Differential Equations

John Travis

Mississippi College

Blanchard, Devaney and Hall, 4th edition.

Once again, we consider various differential equations in the form

$\frac{\mathrm{dy} }{\mathrm{dt}} = f(t,y)$

Qualitative analysis can be appropriate when:

• it may be more difficult to discern an analytic approach to solving the equation
• all you want is an idea about how solutions behave over time
• you need only asymptotic behavior (as $t \to \infty$)
# Worksheet still under development. Use at your own risk! 8-o
t,y,yy = var('t y yy') def f(t,y): # enter the desired right hand side below return t^2+y def ff(t,yy): # enter the desired right hand side below return t^2+yy html('Using $f(t,y) = '+str(f(t,y))+'$') yy = function('yy',t) DE = diff(yy,t)== f(t,yy) html('Differential Equation is $%s$'%str(latex(DE)))
 Using $f(t,y) = t^2 + y$Differential Equation is $D\left({\rm yy}\right)\left(t\right) = t^{2} + {\rm yy}\left(t\right)$ Using Differential Equation is
tmin=-2 tmax=2 ymin=-2 ymax=2
plot_slope_field(f(t,y),(t,tmin,tmax),(y,ymin,ymax)).show(aspect_ratio=1) html('<center>Slope field for $dy/dt = %s$</center>'%str(f(t,y))) Slope field for $dy/dt = t^2 + y$ Slope field for
# Overlay particular slopes on the slope field above at an interactively chosen initial value. Also draw soln curve. @interact def _(t0 = slider(tmin,tmax,1/10,tmin,label='$t_0$'),y0 = slider(ymin,ymax,1/10,(ymin+ymax)/2,label='$y_0$'), soln_curve=checkbox(default=False,label='Draw Solution?')): G = plot_slope_field(f(t,y),(t,tmin,tmax),(y,ymin,ymax),color='gray') G += point((t0,y0),color='red',size=40) G += plot(f(t0,y0)*(t-t0)+y0,(t,t0-0.2,t0+0.2),color='blue') # could blow up if vertical...need parametric if soln_curve: desolve_rk4(t^2+y^2,y,ics=[t0,y0],end_points=[tmin,tmax],step=0.1) # G += points(desolve_rk4(ff(t,yy),yy,ics=[t0,y0],end_points=[tmin,tmax],step=0.1)) G.show(aspect_ratio=1) html('<center>Slope field for $dy/dt = %s$</center>'%str(f(t,y)))

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# The following still have issues. Fixes?
# This works ok so long as DE is autonomous. Putting any t's in the DE make this take forever. P=desolve_rk4(DE,y,ics=[0,1],ivar=t,output='slope_field',end_points=[-4,6],thickness=3) show(P)
points(desolve_rk4(f(t,y),y,ics=[0,1]),ivar=t)
 verbose 0 (154: primitive.py, options) WARNING: Ignoring option 'ivar'=t verbose 0 (154: primitive.py, options) The allowed options for Point set defined by 101 point(s) are: alpha How transparent the point is. faceted If True color the edge of the point. hue The color given as a hue. legend_color The color of the legend text legend_label The label for this item in the legend. rgbcolor The color as an RGB tuple. size How big the point is (i.e., area in points^2=(1/72 inch)^2). zorder The layer level in which to draw verbose 0 (154: primitive.py, options) WARNING: Ignoring option 'ivar'=t verbose 0 (154: primitive.py, options) The allowed options for Point set defined by 101 point(s) are: alpha How transparent the point is. faceted If True color the edge of the point. hue The color given as a hue. legend_color The color of the legend text legend_label The label for this item in the legend. rgbcolor The color as an RGB tuple. size How big the point is (i.e., area in points^2=(1/72 inch)^2). zorder The layer level in which to draw verbose 0 (154: primitive.py, options) WARNING: Ignoring option 'ivar'=t verbose 0 (154: primitive.py, options) The allowed options for Point set defined by 101 point(s) are: alpha How transparent the point is. faceted If True color the edge of the point. hue The color given as a hue. legend_color The color of the legend text legend_label The label for this item in the legend. rgbcolor The color as an RGB tuple. size How big the point is (i.e., area in points^2=(1/72 inch)^2). zorder The layer level in which to draw verbose 0 (154: primitive.py, options) WARNING: Ignoring option 'ivar'=t verbose 0 (154: primitive.py, options) The allowed options for Point set defined by 101 point(s) are: alpha How transparent the point is. faceted If True color the edge of the point. hue The color given as a hue. legend_color The color of the legend text legend_label The label for this item in the legend. rgbcolor The color as an RGB tuple. size How big the point is (i.e., area in points^2=(1/72 inch)^2). zorder The layer level in which to draw verbose 0 (154: primitive.py, options) WARNING: Ignoring option 'ivar'=t verbose 0 (154: primitive.py, options) The allowed options for Point set defined by 101 point(s) are: alpha How transparent the point is. faceted If True color the edge of the point. hue The color given as a hue. legend_color The color of the legend text legend_label The label for this item in the legend. rgbcolor The color as an RGB tuple. size How big the point is (i.e., area in points^2=(1/72 inch)^2). zorder The layer level in which to draw verbose 0 (154: primitive.py, options) WARNING: Ignoring option 'ivar'=t verbose 0 (154: primitive.py, options) The allowed options for Point set defined by 101 point(s) are: alpha How transparent the point is. faceted If True color the edge of the point. hue The color given as a hue. legend_color The color of the legend text legend_label The label for this item in the legend. rgbcolor The color as an RGB tuple. size How big the point is (i.e., area in points^2=(1/72 inch)^2). zorder The layer level in which to draw # do something with isoclines...
# Playing around with slope fields @interact def _(t0 = slider(tmin,tmax,1/10,tmin,label='$t_0$'),y0 = slider(ymin,ymax,1/10,(ymin+ymax)/2,label='$y_0$'), soln_curve=checkbox(default=False,label='Draw Solution?')): G = plot_slope_field(f(t,y),(t,tmin,tmax),(y,ymin,ymax),color='gray') G += point((t0,y0),color='red',size=40) G += plot(f(t0,y0)*(t-t0)+y0,(t,t0-0.2,t0+0.2),color='blue') # could blow up if vertical...need parametric if soln_curve: desolve_rk4(t^2+y,y,ics=[t0,y0],end_points=[tmin,tmax],step=0.1) G += points(desolve_rk4(ff(t,yy),yy,ics=[t0,y0],end_points=[tmin,tmax],step=0.1)) G.show(aspect_ratio=1) html('<center>Slope field for $dy/dt = %s$</center>'%str(f(t,y)))

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