# Analytical Solutions and Differential Equations

The dynamical problem types specify the dynamical models that are the nonlinear transformation of the NLME model. There are two major types of dynamical models: analytical models and `DEProblem`

s. An analytical model is a small differential equation with an analytical solution. This analytical solution is used by the solvers to greatly enhance the performance. On the other hand, `DEProblem`

is a specification of a differential equation for numerical solution by `DifferentialEquations`

. This is used for specifying dynamical equations which do not have an analytical solution, such as many nonlinear ordinary differential equations (ODEs), or the myriad of differential equation types supported by `DifferentialEquations`

, such as delay differential equations (DDEs) and stochastic differential equations (SDEs).

## Analytical Solutions

Analytical problems are a predefined ODE with an analytical solution. While limited in flexibility, the analytical solutions can be much faster for simulation and estimation. In the `@model`

DSL, an analytical solution is declared by name. For example:

`@dynamics Central1`

declares the use of `Central1`

dynamics. Analytical solutions have preset names which are used in the internal model. These parameters must be given values in the `@pre`

block.

`Central1`

The `Central1`

model corresponds to the following `@dynamics`

block:

```
@dynamics begin
Central' = -(CL / Vc) * Central
end
```

The variables `CL`

and `Vc`

are required to be defined in the `@pre`

block.

`Depots1Central1`

The `Depots1Central1`

model corresponds to the following `@dynamics`

block:

```
@dynamics begin
Depot' = -Ka * Depot
Central' = Ka * Depot - (CL / Vc) * Central
end
```

The variables `Ka`

, `CL`

and `Vc`

are required to be defined in the `@pre`

block.

`Depots2Central1`

The `Depots2Central1`

model corresponds to the following `@dynamics`

block:

```
@dynamics begin
Depot1' = -Ka1 * Depot1
Depot2' = -Ka2 * Depot2
Central' = Ka1 * Depot1 + Ka2 * Depot2 - (CL / Vc) * Central
end
```

`Central1Periph1`

The `Central1Periph1`

model corresponds to the following `@dynamics`

block:

```
@dynamics begin
Central' = -(CL + Q) / Vc * Central + Q / Vp * Peripheral
Peripheral' = Q / Vc * Central - Q / Vp * Peripheral
end
```

The variables `CL`

, `Vc`

, `Q`

, and `Vp`

are required to be defined in the `@pre`

block.

`Depots1Central1Periph1`

The `Depots1Central1Periph1`

model corresponds to the following `@dynamics`

block:

```
@dynamics begin
Depot' = -Ka * Depot
Central' = Ka * Depot - (CL + Q) / Vc * Central + Q / Vp * Peripheral
Peripheral' = Q / Vc * Central - Q / Vp * Peripheral
end
```

The variables `Ka`

, `CL`

, `Vc`

, `Q`

, and `Vp`

are required to be defined in the `@pre`

block.

`Central1Periph1Meta1`

The `Central1Periph1Meta1`

model corresponds to the following `@dynamics`

block:

```
@dynamics begin
Central' = -(CL + Q + CLfm) / Vc * Central + Q / Vp * CPeripheral
CPeripheral' = Q / Vc * Central - Q / Vp * CPeripheral
Metabolite' = -CLm / Vm * Metabolite + CLfm / Vc * Central
end
```

The variables `CL`

, `CLm`

, `Vc`

, `Vp`

, `Vm`

, `Q`

, and `CLfm`

are required to be defined in the `@pre`

block.

`Central1Periph1Meta1Periph1`

The `Central1Periph1Meta1Periph1`

model corresponds to the following `@dynamics`

block:

```
@dynamics begin
Central' = -(CL + Q + CLfm) / Vc * Central + Q / Vp * CPeripheral
CPeripheral' = Q / Vc * Central - Q / Vp * CPeripheral
Metabolite' = -(CLm + Qm) / Vm * Metabolite + Qm / Vmp * MPeripheral + CLfm / Vc * Central
MPeripheral' = Qm / Vm * Metabolite - Qm / Vmp * MPeripheral
end
```

The variables `CL`

, `CLm`

, `Vc`

, `Vp`

, `Vmp`

, `Q`

, `Qm`

, and `CLfm`

are required to be defined in the `@pre`

block.

## Differential Equations `DEProblem`

`DEProblem`

s are types from `DifferentialEquations`

which are used to specify differential equations to be solved numerically via the solvers of the package. In the `@model`

interface, the `DEProblem`

is set to be an `ODEProblem`

defining an ODE. The models are defined by writing each of the differential equations in the system, e.g.

```
@dynamics begin
Central' = -(CL / Vc) * Central
end
```

for a simple one compartment model and

```
@dynamics begin
Depot' = -Ka * Depot
Central' = Ka * Depot - (CL / Vc) * Central
end
```

for a one compartment model with first order absorption.