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ParameterizedFunctions.jl is a component of the SciML ecosystem which allows for easily defining parameterized ODE models in a simple syntax.

Basic Usage

ODE Macros

A helper macro is provided to make it easier to define a ParameterizedFunction, and it will symbolically compute a bunch of extra functions to make the differential equation solvers run faster. For example, to define the previous LotkaVolterra, you can use the following command:

f = @ode_def LotkaVolterra begin
  dx = a*x - b*x*y
  dy = -c*y + d*x*y
end a b c d

or you can define it anonymously:

f = @ode_def begin
  dx = a*x - b*x*y
  dy = -c*y + d*x*y
end a b c d

The macro also defines the Jacobian f'. This is defined as an in-place Jacobian f(Val{:jac},t,u,J). This is calculated using SymEngine.jl automatically, so it's no effort on your part. The symbolic inverse of the Jacobian is also computed, and an in-place function for this is available as well as f(Val{:invjac},t,u,iJ). If the Jacobians cannot be computed, a warning is thrown and only the function itself is usable. The functions jac_exists(f) and invjac_exists(f) can be used to see whether the Jacobian and the function for its inverse exist.

Extra Options

In most cases the @ode_def macro should be sufficient. This is because by default the macro will simply calculate each function symbolically, and if it can't it will simply throw a warning and move on. However, in extreme cases the symbolic calculations may take a long time, in which case it is necessary to turn them off. To do this, use the ode_def_opts function. The @ode_def macro simply defines the specifiable options:

opts = Dict{Symbol,Bool}(
      :build_tgrad => true,
      :build_jac => true,
      :build_expjac => false,
      :build_invjac => true,
      :build_invW => true,
      :build_invW_t => true,
      :build_hes => false,
      :build_invhes => false,
      :build_dpfuncs => true)

and calls the function ode_def_opts(name::Symbol,opts,ex::Expr,params). Note that params is an iterator holding expressions for the parameters.

In addition, one can also use their own function inside of the macro. For example:

f(x,y,d) = erf(x*y/d)
NJ = @ode_def FuncTest begin
  dx = a*x - b*x*y
  dy = -c*y + f(x,y,d)
end a b c d

will do fine. The symbolic derivatives will not work unless you define a derivative for f.

Extra Macros

Instead of using ode_def_opts directly, one can use one of the following macros to be more specific about what to not calculate. In increasing order of calculations:


Extra Functions

Jacobian Function

The Jacobian overload is provided by overloading in the following manner:

function (p::LotkaVolterra)(::Type{Val{:jac}},t,u,J)
  J[1,1] = p.a - p.b * u[2]
  J[1,2] = -(p.b) * u[1]
  J[2,1] = 1 * u[2]
  J[2,2] = -3 + u[1]

Inverse Jacobian

The Inverse Jacobian overload is provided by overloading in the following manner:

function (p::LotkaVolterra)(::Type{Val{:invjac}},t,u,J)
  J[1,1] = (1 - (p.b * u[1] * u[2]) / ((p.a - p.b * u[2]) * (-3 + u[1] + (p.b * u[1] * u[2]) / (p.a - p.b * u[2])))) / (p.a - p.b * u[2])
  J[1,2] = (p.b * u[1]) / ((p.a - p.b * u[2]) * (-3 + u[1] + (p.b * u[1] * u[2]) / (p.a - p.b * u[2])))
  J[2,1] = -(u[2]) / ((p.a - p.b * u[2]) * (-3 + u[1] + (p.b * u[1] * u[2]) / (p.a - p.b * u[2])))
  J[2,2] = (-3 + u[1] + (p.b * u[1] * u[2]) / (p.a - p.b * u[2])) ^ -1

Parameter Jacobian

For solvers which need parameters derivatives, specifying the functions can increase performance. For our example, we allow the solvers to use the explicit derivatives in the parameters by:

function (p::LotkaVolterra)(::Type{Val{:paramjac}},J,u,p,t)
    J[1, 1] = u[1] * 1
    J[1, 2] = -(u[1]) * u[2]
    J[1, 3] = 0 * 1
    J[1, 4] = 0 * 1
    J[2, 1] = 0 * 1
    J[2, 2] = 0 * 1
    J[2, 3] = -(u[2])
    J[2, 4] = u[1] * u[2]


A simple domain-specific language (DSL) for defining differential equations for use in scientific machine learning (SciML) and other applications




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