# DiffEqFunctions (Jacobians, Gradients, etc.) and Jacobian Types

The DiffEq ecosystem provides an extensive interface for declaring extra functions associated with the differential equation's data. In traditional libraries there is usually only one option: the Jacobian. However, we allow for a large array of pre-computed functions to speed up the calculations. This is offered via the DiffEqFunction types which can be passed to the problems.

## Function Type Definitions

### Function Choice Definitions

The full interface available to the solvers is as follows:

• jac: The Jacobian of the differential equation with respect to the state variable u at a time t with parameters p.
• tgrad: The gradient of the differential equation with respect to t at state u with parameters p.
• paramjac: The Jacobian of the differential equation with respect to p at state u at time t.
• analytic: Defines an analytical solution using u0 at time t with p which will cause the solvers to return errors. Used for testing.
• Wfact: The LU-factorization of M - gamma*J where J is the jac.
• Wfact_t: The LU-factorization of M/gamma - J where J is the jac.
• ggprime: See the definition in the SDEProblem page.
• syms: Allows you to name your variables for automatic names in plots and other output.

### ODEFunction

function ODEFunction{iip,recompile}(f;
mass_matrix=I,
analytic=nothing, # (u0,p,t)
jac=nothing, # (J,u,p,t) or (u,p,t)
jac_prototype=nothing, # Type for the Jacobian
Wfact=nothing, # (iW,u,p,gamma,t) or (u,p,gamma,t)
Wfact_t=nothing, # (iW,u,p,gamma,t) or (u,p,gamma,t)
paramjac = nothing, # (pJ,u,p,t) or (u,p,t)
colorvec = nothing,
syms = nothing) # collection of names for variables

### DynamicalODEFunction

DynamicalODEFunction{iip,recompile}(f1, # (du,u,v,p,t) or (u,v,p,t)
f2; # (du,u,v,p,t) or (u,v,p,t)
mass_matrix=(I,I), # Mass matrix for each partition
analytic=nothing)

### SplitFunction

SplitFunction{iip,recompile}(f1, # ODEFunction
f2; # ODEFunction
mass_matrix=I,
_func_cache=nothing, # This is a cache used in f = f1+f2
analytic=nothing)

### SDEFunction

function SDEFunction{iip,recompile}(f,g;
mass_matrix=I,
analytic=nothing,
jac=nothing,
jac_prototype=nothing,
Wfact=nothing,
Wfact_t=nothing,
paramjac = nothing,
ggprime = nothing,
colorvec = nothing,
syms = nothing)

Notice here that the Jacobian refers to the Jacobian of the drift function f. The sparsity, colorvec, etc. all refer to this Jacobian. ggprime refers to g(u,p,t)*Dg(u,p,t).

### SplitSDEFunction

SplitSDEFunction{iip,recompile}(f1, # ODEFunction
f2, # ODEFunction
g;
mass_matrix=I,
_func_cache=nothing,
analytic=nothing)

### RODEFunction

function RODEFunction{iip,recompile}(f;
mass_matrix=I,
analytic=nothing,
jac=nothing,
jac_prototype=nothing,
Wfact=nothing,
Wfact_t=nothing,
paramjac = nothing,
colorvec = nothing,
syms = nothing)

### DAEFunction

function DAEFunction{iip,recompile}(f;
mass_matrix=I,
analytic=nothing,
jac=nothing, # (J,du,u,p,gamma,t) or (du,u,p,gamma,t)
jac_prototype=nothing,
Wfact=nothing,
Wfact_t=nothing,
paramjac = nothing,
syms = nothing)

Note that the Jacobian of a DAE is defined as gamma*dG/d(du) + dG/du where gamma is given by the solver.

### DDEFunction

function DDEFunction{iip,recompile}(f;
mass_matrix=I,
analytic=nothing,
jac=nothing,
jac_prototype=nothing,
Wfact=nothing,
Wfact_t=nothing,
paramjac = nothing,
colorvec = nothing,
syms = nothing)

## Inplace Specification and No-Recompile Mode

Each DiffEqFunction type can be called with an "is inplace" (iip) choice.

ODEFunction(f)
ODEFunction{iip}(f)

which is a boolean for whether the function is in the inplace form (mutating to change the first value). This is automatically determined using the methods table but note that for full type-inferrability of the DEProblem this iip-ness should be specified.

Additionally, the functions are fully specialized to reduce the runtimes. If one would instead like to not specialize on the functions to reduce compile time, then one can set recompile to false.

ODEFunction{iip,false}(f)

This makes the ODE solver compilation independent of the function and so changing the function will not cause recompilation. One can change the default value by changing the const RECOMPILE_BY_DEFAULT = true to false in the DiffEqBase.jl source code.

## Specifying Jacobian Types

The jac field of an inplace style DiffEqFunction has the signature jac(J,u,p,t), which updates the jacobian J in-place. The intended type for J can sometimes be inferred (e.g. when it is just a dense Matrix), but not in general. To supply the type information, you can provide a jac_prototype in the function's constructor.

The following example creates an inplace ODEFunction whose jacobian is a Diagonal:

using LinearAlgebra
f = (du,u,p,t) -> du .= t .* u
jac = (J,u,p,t) -> (J[1,1] = t; J[2,2] = t; J)
jp = Diagonal(zeros(2))
fun = ODEFunction(f; jac=jac, jac_prototype=jp)

Note that the integrators will always make a deep copy of fun.jac_prototype, so there's no worry of aliasing.

In general the jacobian prototype can be anything that has mul! defined, in particular sparse matrices or custom lazy types that support mul!. A special case is when the jac_prototype is a AbstractDiffEqLinearOperator, in which case you do not need to supply jac as it is automatically set to update_coefficients!. Refer to the DiffEqOperators section for more information on setting up time/parameter dependent operators.

## Examples

### Declaring Explicit Jacobians for ODEs

The most standard case, declaring a function for a Jacobian is done by overloading the function f(du,u,p,t) with an in-place updating function for the Jacobian: f_jac(J,u,p,t) where the value type is used for dispatch. For example, take the LotkaVolterra model:

function f(du,u,p,t)
du = 2.0 * u - 1.2 * u*u
du = -3 * u + u*u
end

To declare the Jacobian we simply add the dispatch:

function f_jac(J,u,p,t)
J[1,1] = 2.0 - 1.2 * u
J[1,2] = -1.2 * u
J[2,1] = 1 * u
J[2,2] = -3 + u
nothing
end

Then we can supply the Jacobian with our ODE as:

ff = ODEFunction(f;jac=f_jac)

and use this in an ODEProblem:

prob = ODEProblem(ff,ones(2),(0.0,10.0))

### Declaring Explicit Jacobians for DAEs

For fully implicit ODEs (DAEProblems), a slightly different Jacobian function is necessary. For the DAE

$G(du,u,p,t) = res$

The Jacobian should be given in the form gamma*dG/d(du) + dG/du where gamma is given by the solver. This means that the signature is:

f(J,du,u,p,gamma,t)

For example, for the equation

function testjac(res,du,u,p,t)
res = du - 2.0 * u + 1.2 * u*u
res = du -3 * u - u*u
end

we would define the Jacobian as:

function testjac(J,du,u,p,gamma,t)
J[1,1] = gamma - 2.0 + 1.2 * u
J[1,2] = 1.2 * u
J[2,1] = - 1 * u
J[2,2] = gamma - 3 - u
nothing
end

## Symbolically Calculating the Functions

See the modelingtoolkitize function from ModelingToolkit.jl for automatically symbolically calculating the Jacobian for numerically-defined functions.