XPP - EXAMPLES

Contents

# Examples

#### Systems of ODEs

#### Higher order ODEs

#### Maps and difference equations

#### Delay equations

#### Differential-algebraic equations

#### Volterra equations

#### Markov models

#### Delta functions

#### Boundary value problems

#### Discretized PDEs

#### Spatial networks

- ### Differential equation

  Here is the Wilson-Cowan equation:

  u' = -u+f(aeeu-aiev-ze + Ie(t) )
  v' = (-v+f(aeiu-aiiv-zi + Ii(t) ))/tau

  and here is the corresponding ODE file:

      # wilson-cowan
      f(u)=1/(1+exp(-u))
      par aee=10,aie=8,aei=12,aii=3,ze=.2,zi=4
      par tau=1
      i_e(t)=ie0+ie1*sin(w*t)
      i_i(t)=ii0+ii1*sin(w*t)
      par ie0=0,ie1=0,w=.25
      par ii0=0,ii1=0
      init u=.1,v=.05
      u'=-u+f(aee*u-aie*v-ze+i_e(t))
      v'=(-v+f(aei*u-aii*v-zi+i_i(t)))/tau
      @ total=40
      @ xp=u,yp=v,xlo=-.1,xhi=1,ylo=-.1,yhi=1
      done

  I have added definitions for the inputs and the function f as well as default initial data and parameters. I also set it up so that the initial window is the phaseplane.

- ### Higher order differential equations

  XPP cannot solve higher order equations. You must first write them as a system of first order equations. For example, the third order equation:

  x''' + a x'' + b x' + c x = x2

  should be rewritten as

  x' = y
  y' = z
  z' = x2 - a z -b y -c x

  with the corresponding ODE file

      # third.ode
      x'=y
      y'=z
      z'=x^2-a*z-b*y-c*x
      par a=1,b=2,c=3.7
      init x=1,y=0,z=0
      aux en=x^2+y^2+z^2
      @ total=200
      @ xp=x,yp=y,xlo=-2,xhi=5,ylo=-4,yhi=5
      done

  I have set the plotting parameters and integration time. I have also added another quantity en which is stored and can be plotted as well.

- ### Difference equation or maps

  The example is the delayed logistic map

  xn+1 = a xn-1(1-xn)

  This is a second order map and XPP can only solve first order equations. We write it as a pair of first order equations:

  xn+1 = yn
  yn+1 = a yn(1-xn)

  and get the corresponding ODE file:

      # delayed logistic map
      x'=y
      y'=a*y*(1-x)
      par a=2.2
      init x=.2,y=.2
      # set method to discrete
      @ meth=discrete
      @ total=100
      @ ylo=-.1,yhi=1,xhi=100
      done

  Here the most important thing is that I tell XPP that this is a discrete dynamical system.

- ### Delay-differential equation

  XPP can solve delay equations with one or more delays. Here is a delay equation due to Mike Mackey and Leon Glass:

  x'(t) = -g x(t) + b f(x(t-d))
  f(x) = x/(1+xn)

  Here is the ODE file:

      # mackey-glass
      f(x)=x/(1+x^n)
      x'=-g*x+b*f(delay(x,d))
      init x=.2
      par g=.1,b=.2,n=10,d=6
      @ delay=20
      @ total=200
      @ xhi=200,yhi=2
      done

  I have told XPP that the maximal delay to expect is 20.

  Delay equations with delay d require initial data on an interval -d < t < 0. You can set these in the ODE file by adding the line:

      x(0)=exp(t)*0.2

  which sets x(t)=0.2et for -d < t < 0.

  NOTE this does not set the initial condition for x at t=0. You can do this by adding one more line after the delayed condition:

      x(0)=exp(t)*0.2
      init x=.2

  #### Setting delay initial data within XPP

  There is a window called Delay ICs which shows up in XPP and you can use this to type in delay initial data. Before integrating delay equations always click on the OK button in the Delay ICs window! Otherwise, the delay initial data will not take effect.

- ### Differential-algebraic equations

  Differential algebraic equations of the form:

  x' = F(x,y)
  0 = G(x,y)

  can be solved in XPP. The DAE

  x' = z
  z' = y
  0 = x+z+y+y^3

  has an ODE file:

      # dae.ode
      x'=z
      z'=y
      0=x+z+y+y^3
      solve y=-1
      init x=2
      aux ys=y
      @ yhi=2.5
      done

  You have to tell it which variables to solve for. In this case, we solve for y as a function of x,z . You can give XPP a guess about what the value of y is at the initial conditions for x,z. I have added an additional quantity ys which is the solution to the algebraic condition.

- ### Volterra integral equations

  XPP solves both continuous and singular integral equations. Here is a regular equation:

  u(t)=sin(t) + 0.5 cos(t)-0.5 e-t(t + 1) + int[u(t') (t-t') et'-t,t'=0..t]

  and here is the ODE file

      # volterra example 1
      u(t)=sin(t)+.5*cos(t)-.5*t*exp(-t)-.5*exp(-t)+int{(t-t')*exp(t'-t)*u}
      aux utrue=sin(t)
      done

  I have included the true solution as well. This is actually a convolution equation so that you and improve the speed by exploiting that:

      #  volt2.ode - uses convolution to speed up soln
      u(t)=sin(t)+.5*cos(t)-.5*t*exp(-t)-.5*exp(-t)+int{t*exp(-t)#u}
      aux utrue=sin(t)
      done

  Integro-differential equations are also possible:

  u'(t)=-2u+e-2t + int[u^2(t')et'-t,t'=0..t]

  with u(0)=1 has a solution u(t)=e-t as can be confirmed with the ODE file

      # nonlinear integrodifferential volterra equation
      u'=-2*u+exp(-2*t)+int{exp(-t)#u^2}
      init u=1
      aux utrue=exp(-t)
      done

  Finally, integral equations with singularities are easily solved. For example

  u(t)=exp(-t)-int[e-(t-s)(t-s)-¼ u(s),s=0..t]

  This has a removeable singularity in that the power of t is less than 1. The ODE file has the form:

      # singular integral equation
      u(t)=exp(-t)-int[.25]{exp(-t)#sin(u)}
      done

  WARNINGS

  1. The expression inside the [ ] must be a number and not a parameter.
  2. Volterra integral equations cannot be edited inside XPP.

- ### Markov models

  XPP simulates random processes. Here is a stochastic ratchet:

  dx=z f(x) dt + s dW

  The variable z randomly switches back and forth between 0 and 1 at rates r0,r1 respectively. dW is a Wiener process. The function f(x) is an asymmetric periodic potential with zero mean. Here is the ODE file

      # a simple thermal ratchet
      wiener w
      par a=.8,s=.05,r0=.1,r1=.1
      f(x)=if(x<a)then(-1)else(a/(1-a))
      x'=z*f(mod(x,1))+s*w
      # z is two states
      markov z 2
      {0} {r0}
      {r1} {0}
      @ meth=euler,dt=.1,total=2000,nout=10
      @ xhi=2000,yhi=8,ylo=-8
      done

  The rates in a Markov process need not be constant but can depend on any of the variables or parameters. Always use Euler's method for stochastic equations.

- ### Delta functions and all that

  XPP has a way of dealing with delta-function discontinuities. XPP uses global flags which check for events and act on variables accordingly. Here is an equation due to John Tyson for the cell cycle:

  u' = k4 (v-u)(a + u2) - k6
  v' = k1 -k6 u
  m' = b m

  with the condition that when u falls below 0.2 the mass is halved. We tell XPP to look for crossings of u and to cut the mass in half. Here is the ODE file

      # tyson.ode
      init u=.0075,v=.48,m=1
      p k1=.015,k4=200,k6=2,a=.0001,b=.005
      u'=  k4*(v-u)*(a+u^2) - k6*u
      v'= k1*m - k6*u
      m'= b*m
      global 1 {0.2-u} {m=.5*m}
      @ total=1000,nout=10
      @ yp=m,xhi=1000,ylo=0,yhi=2
      done

- ### Boundary value problems

  Boundary value problems require that you satisfy conditions at both ends of the integration interval. Consider the boundary value problem:

  u'' = sin(t u)
  u(0)= 1
  u(2) = 0

  We rewrite this as a first order system:

  u' = v
  v' = sin(t u)
  u(0)= 1
  u(2) = 0

  Here is the ODE file for this:

      # nonlinear boundary value problem
      u'=v
      v'=sin(t*u)
      bdry u-1
      bdry u'
      init u=1,v=-1
      @ total=2,bell=0,xhi=2
      done

  Boundary conditions take the form:

      bdry F(u,u')

  where u' represents the value of the variable at the end of the interval. It is not the derivative in this context!! XPP tries to make the functions defined by each boundary condition vanish. The length of the interval is the total amount of time integrated. Note that in this example, I have turned off the stinking bell.

  You solve boundary value problems in XPP by using the Bdryval menu item instead of the Initialconds item.

  XPP can also solve systems with homoclinic orbits. See the manual for an example.

- ### Discretized PDEs

  XPP can make fake arrays of variables and save you from typing every one. The Schnakenberg system is a classic model:

  ut = -u+vu2+du uxx
  vt = a - vu2+dv vxx

  which we can discretize yielding:

  u'j = -uj+vju2j+(du/h2) (uj+1-2 uj+uj-1)
  v'j = a - vju2j+(dv/h2) (vj+1-2 vj+vj-1)

  with appropriate end conditions. We will make this into an ODE file:

      # schnakenberg PDE 100 points
      f(u,v)=-u+v*u^2
      g(u,v)=a-v*u^2
      u0'=f(u0,v0)+duh*(u1-u0)
      u[1..99]'=f(u[j],v[j])+duh*(u[j+1]-2*u[j]+u[j-1])
      u100'=f(u100,v100)+duh*(u99-u100)
      v0'=g(u0,v0)+dvh*(v1-v0)
      v[1..99]'=g(u[j],v[j])+dvh*(v[j+1]-2*v[j]+v[j-1])
      v100'=g(u100,v100)+dvh*(v99-v100)
      par a=1.05,du=.2,dv=3,h=.2
      !duh=du/(h*h)
      !dvh=dv/(h*h)
      init u0=1.5,u1=1.3,u2=1.1,v0=1.05,v1=1.05,v2=1.05
      init u[3..100]=1.05,v[j]=1.05
      @ dt=.1,nout=50,total=50,meth=cvode,tol=1e-5,atol=1e-5
      done

  Note that I have defined the kinetics as a pair of functions. The statements starting with the ! are derived parameters -- parameters which depend on other parameters. I use a stiff integrator CVODE . Plot this using array plots or look at the spatial profile with the transpose operation.

  Take advantage of the banded quality of the system and speed up the stiff integrators my an order of magnitude. You have to rearrange the equations a bit. Here is the ODE file:

      # schnakenberg PDE 100 points
      # interlaced
      f(u,v)=-u+v*u^2
      g(u,v)=a-v*u^2
      u0'=f(u0,v0)+duh*(u1-u0)
      v0'=g(u0,v0)+dvh*(v1-v0)
      %[1..99]
      u[j]'=f(u[j],v[j])+duh*(u[j+1]-2*u[j]+u[j-1])
      v[j]'=g(u[j],v[j])+dvh*(v[j+1]-2*v[j]+v[j-1])
      %
      u100'=f(u100,v100)+duh*(u99-u100)
      v100'=g(u100,v100)+dvh*(v99-v100)
      par a=1.05,du=5,dv=75,h=.2
      !duh=du/(h*h)
      !dvh=dv/(h*h)
      init u0=1.5,u1=1.3,u2=1.1,v0=1.05,v1=1.05,v2=1.05
      init u[3..100]=1.05,v[j]=1.05
      @ dt=.1,nout=50,total=50,meth=cvode,tol=1e-5,atol=1e-5
      @ bandlo=2,bandup=2
      done

  I tell XPP the system is banded and interlace the two variables. This runs significantly faster than the previous ODE!

- ### Network problems and tables

  The last example is a solution to the Wilson-Cowan equations distributed in space. We will solve:

  u'j = -uj + F(aeeKe,j -aie Ki,j)
  v'j = (-vj + F(aeiKe,j -aii Ki,j))/tau

  where

  Ke,j = SUM(We(k)uj-k,k=-m..m)
  Ki,j = SUM(Wi(k)uj-k,k=-m..m)

  with some sort of "boundary" conditions at the edges. Here is an XPP file for a network of 201 cells and an exponential weight function:

      # Wilson-Cowan network
      table we % 51 -25 25 .5*exp(-abs(t)/se)/se
      table wi % 51 -25 25 .5*exp(-abs(t)/si)/si
      special ke=conv(even,201,25,we,u0)
      special ki=conv(even,201,25,wi,v0)
      par se=4,si=2.5
      f(u)=1/(1+exp(-u))
      par aee=16,aie=10,aei=25,aii=3,ze=4,zi=10
      par tau=4
      init u[0..4]=1
      u[0..200]'=-u[j]+f(aee*ke([j])-aie*ki([j])-ze)
      v[0..200]'=(-v[j]+f(aei*ke([j])-aii*ki([j])-zi))/tau
      @ total=60,meth=qualrk,dt=.25
      @ yp=u100,xhi=60,ylo=0
      @ yp2=u150,nplot=2
      done

  The weights are defined as tables which give the number of points in the table, the x-range, and the functional form of the table. You can also read a file to fill a table. Consult the documentation for more. The special declaration convolves the values in the tables with the variables. The even means that the ends are reflected. Note that you must write ke([j]), that is the [ ] must be included. I have also told XPP to plot two things, u100 and u150.