Ordinary Differential Equations (ODEs) describe the rate of change of dependent variables with respect to a single independent variable and are used in many fields to model behavior of the system. There are many good `C`

libraries available to solve (i.e., integrate systems of ODEs) and SUNDIALS available from the Lawrence Livermore National Laboratory is a one of the most popular and well-respected `C`

library for solving non-stiff and stiff systems of ODEs.

Currently, this package provides an interface to the `CVODE`

and `CVODES`

function (serial version) in the library which is used to solve ODEs (or Initial Value Problems) and calculate sensitivities.

The two exported functions from the package are:

`CVODE`

- An interface to the`CVODE`

function in`SUNDIALS`

to solve a system of ODEs.`CVODES`

- An interface to the`CVODES`

function in`SUNDIALS`

to calculate forward sensitivites with respect to parameters of the ODE system.

In future, we plan to provide interface for the other solvers (i.e., `IDA/IDAS`

and `ARCODE`

in the library also. Right now, this package serves as a test case for providing an interface to the `SUNDIALS`

library for `R`

users.

*One of the advantage of using this package is that all the source code of the SUNDIALS library is bundled with the package itself, so it does not require the SUNDIALS library to be installed on the machine separately (which is sometimes non trivial on a Windows machine).*

As described in the link above, the problem is from chemical kinetics, and consists of the following three rate equations:

\[ \begin{aligned} \frac{dy_1}{dt} &= -.04 \times y_1 + 10^4 \times y_2 \times y_3 \\ \frac{dy_2}{dt} &= .04 \times y_1 - 10^4 \times y_2 \times y_3 - 3 \times 10^7 \times y_2^2 \\ \frac{dy_3}{dt} &= 3 \times 10^7 \times y_2^2 \end{aligned} \]

with time interval from \(t = 0.0\) to \(t = 4 \times 10^{10}\) and initial conditions: \[ y_1 = 1.0 , ~y_2 = y_3 = 0 \]

**The problem is stiff.**

The original example , While integrating the system, also uses the rootfinding feature to find the points at which

\[ y_1 = 1 \times 10^{-4} \] or at which \[ y_3 = 0.01 \] but currently root-finding is not supported in this version. As in the original example, this package also solves the problem with the `BDF`

method, Newton iteration with the `SUNDENSE`

dense linear solver, however, without a user-supplied Jacobian routine (unlike the original example). The future versions may include an ability to provide Jacobian calculated analytically or via automatic differentiation. `CVODE`

uses a scalar relative tolerance and a vector absolute tolerance (which can be provided as an input). Output is printed in decades from \(t = 0.4\) to \(t = 4 \times 10^{10}\) in this example.

Differential equations can be written as an `R`

function or as an `Rcpp`

function. Differential equations function must be written as

```
function(t, y, p){
# code to write differential equations
# using parameter vector (p) and state/entity vector (y)
# should return `ydot`, the vector representing
# rate of change of entities in `y`
# length of `ydot` must be equal to `y1
}
```

where `t`

represents time, `y`

is the vector describing the values of states/entities of the ODE system at time `t`

and `p`

is the vector of parameters used to define the ODEs. The output of this function is a vector of rate of change of entities of `y`

.

The key aspect to keep in mind is that the signature of the function must be `function(t,y,p)`

. As an example, we try to solve the `cv_Roberts_dns.c`

problem described above, the original code can be found here. An example of an `R`

function is as follows:

```
ODE_R <- function(t, y, p){
## initialize the derivative vector
ydot <- vector(mode = "numeric", length = length(y))
## p (parameter vector input) is [-0.04 1e04 3e07]
ydot[1] = p[1]*y[1] + p[2]*y[2]*y[3]
ydot[2] = -p[1]*y[1] - p[2]*y[2]*y[3] - p[3]*y[2]*y[2]
ydot[3] = p[3]*y[2]*y[2]
ydot ## return ydot
}
```

where `p`

is a parameter vector with the values `[-0.04 1e04 3e07]`

.

Also, since this package using `Rcpp`

to bundle the C code, we can use the notation used in `Rcpp`

to describe the system of ODEs. The `cv_Roberts_dns`

problem describe above can be described in an `Rcpp`

function as follows (indices in `C++`

start from `0`

, functions need to declare their return type, here `NumericVector`

and every expression ends in a semicolon, `;`

) :

```
#include <Rcpp.h>
using namespace Rcpp;
// [[Rcpp::export]]
NumericVector ODE_Rcpp (double t, NumericVector y){
// Initialize ydot filled with zeros
NumericVector ydot(y.length());
// p (parameter vector) is [-0.04 1e04 3e07]
ydot[0] = p[0] * y[0] + p[1] * y[1] * y[2];
ydot[1] = -p[0]*y[0] - p[1]*y[1]*y[2] - p[2]*y[1]*y[1]
ydot[2] = p[2] * y[1] * y[1];
return ydot;
}
```

The above is a re-write of the `cvRoberts_dns.c`

example in the documentation of `CVODE`

. The original example can be found the document here.

The entire `R`

file to create right hand side of ODE function (which calculates rates of change) is as follows (also found in `/inst/examples/cv_Roberts_dns.r`

):

```
# ODEs described by an R function
ODE_R <- function(t, y, p){
## initialize the derivative vector
ydot <- vector(mode = "numeric", length = length(y))
## p (parameter vector) is [-0.04 1e04 3e07]
ydot[1] = p[1]*y[1] + p[2]*y[2]*y[3]
ydot[2] = -p[1]*y[1] - p[2]*y[2]*y[3] - p[3]*y[2]*y[2]
ydot[3] = p[3]*y[2]*y[2]
ydot ## return ydot
}
# ODEs can also be described using Rcpp
Rcpp::sourceCpp(code = '
#include <Rcpp.h>
using namespace Rcpp;
// [[Rcpp::export]]
NumericVector ODE_Rcpp (double t, NumericVector y){
// Initialize ydot filled with zeros
NumericVector ydot(y.length());
// p (parameter vector) is [-0.04 1e04 3e07]
ydot[0] = p[0] * y[0] + p[1] * y[1] * y[2];
ydot[1] = -p[0]*y[0] - p[1]*y[1]*y[2] - p[2]*y[1]*y[1]
ydot[2] = p[2] * y[1] * y[1];
return ydot;
}')
# Generate time vector, IC and call cvode to solve the equations
# R code to genrate time vector, IC and solve the equations
time_vec <- c(0.0, 0.4, 4.0, 40.0, 4E2, 4E3, 4E4, 4E5, 4E6, 4E7, 4E8, 4E9, 4E10)
IC <- c(1,0,0)
params <- c(0.04, 10000, 30000000)
reltol <- 1e-04
abstol <- c(1e-8,1e-14,1e-6)
## Solving the ODEs using cvode function
df1 <- cvode(time_vec, IC, ODE_R , params, reltol, abstol) ## using R
df2 <- cvode(time_vec, IC, ODE_Rcpp , params, reltol, abstol) ## using Rcpp
## Check that both solutions are identical
# identical(df1, df2)
```

The final output is the `df1`

matrix in which first column is time, second, third and fourth column are the values of `y1`

, `y2`

and `y3`

respectively.

```
> df1
[,1] [,2] [,3] [,4]
[1,] 0e+00 1.000000e+00 0.000000e+00 0.00000000
[2,] 4e-01 9.851641e-01 3.386242e-05 0.01480205
[3,] 4e+00 9.055097e-01 2.240338e-05 0.09446793
[4,] 4e+01 7.158016e-01 9.185043e-06 0.28418924
[5,] 4e+02 4.505209e-01 3.222826e-06 0.54947590
[6,] 4e+03 1.832217e-01 8.943516e-07 0.81677741
[7,] 4e+04 3.898091e-02 1.621669e-07 0.96101893
[8,] 4e+05 4.936971e-03 1.984450e-08 0.99506301
[9,] 4e+06 5.170103e-04 2.069098e-09 0.99948299
[10,] 4e+07 5.204927e-05 2.082078e-10 0.99994795
[11,] 4e+08 5.184946e-06 2.073989e-11 0.99999482
[12,] 4e+09 5.246212e-07 2.098486e-12 0.99999948
[13,] 4e+10 6.043000e-08 2.417200e-13 0.99999994
```

Sensitivity with respect to the parameters of the ODE system can be calculated using `CVODES`

function. This package implements Forward Sensitivity Analysis from `CVODES`

function (see the example `cvRoberts_FSA_dns.c`

from the link here). Briefly, given the ODE system as described below

\[
\begin{aligned}
\frac{dy_1}{dt} &= -p_1y_1 + p_2y_2y_3 \\
\frac{dy_2}{dt} &= p_1y_1 - p_2y_2y_3 - p_3y_2^2 \\
\frac{dy_3}{dt} &= p_3y_2^2
\end{aligned}
\] with the same initial conditions as above (i.e., \(y_1 = 0, y_2 = y_3 = 0\)) and \(p_1 = 0.04, \quad p_2 = 10^4, \quad p_3 = 3\times10^7\). The system of Sensitivity equations (taken from `cvs_guide.pdf`

) that is solved can be given by \[
\begin{aligned}
\frac{ds}{dt} =
\left[\begin{array}
{ccc}
-p_1 & p_2y_3 & p_2y_2 \\
p_1 & -p_2y_3-2p_3y_2 & -p_2y_2 \\
0 & 2p_3y_2 & 0
\end{array}\right]s_i + \frac{\partial f}{\partial p_i},
\quad s_i(t_0) =
\left[\begin{array} {c} 0 \\ 0 \\ 0 \end{array}\right], \quad i = 1, 2, 3
\end{aligned}
\] where \[
\frac{\partial f}{\partial p_1} = \left[\begin{array} {c} -y_1 \\ y_1 \\ 0 \end{array}\right],
\quad \frac{\partial f}{\partial p_2} = \left[\begin{array} {c} y_2y_3 \\ -y_2y_3 \\ 0 \end{array}\right],
\quad
\frac{\partial f}{\partial p_3} = \left[\begin{array} {c} 0 \\ -y_2^2 \\ y_2^2 \end{array}\right]
\] In the original `CVODES`

interface from `SUNDIALS`

, the sensitivity equations can either be provided by the user or can be calculated using numerical interpolation by the solver. Here, I have only included the numerical interpolation version and currently the user cannot specify the sensitivity equations. However, in the future versions I will provide an ability to specify user-defined Jacobian as well as user-defined sensitivity equations.

Also, currently, forward sensitivities are calculated with respect to all parameters of the system. I plan to provide in future, an ability to specify specific particular parameters for which sensitivity is desired. Currently, `SIMULATENOUS`

and `STAGGERED`

methods of sensitivity calculations from the `SUNDIALS`

library are supported in this package.

`CVODES`

Once, the system of ODEs has been defined using the instructions provided above, sensitivities can be easily calculated using the `cvodes`

function using the function call below:

```
df1 <- cvodes(time_vec, IC, ODE_R , params, reltol, abstol,"STG",F) ## using R
df2 <- cvodes(time_vec, IC, ODE_Rcpp , params, reltol, abstol,"STG",F) ## using R
```

The additional arguments in `cvodes`

specify the senstivity calculation method to be used (`STG`

for `STAGGERED`

or `SIM`

for `SIMULATENOUS`

) and flag for error control (either `T`

or `F`

).

The output of `cvodes`

is a matrix with number of rows equal to the length of the time vector (`time_vec`

) and the number of columns being equal to length of (`y`

\(\times\) `p`

+ 1). The first columns is for time. Currently, the sensitiviy of every enitity is calculated with respect to every parameter in model. For example, for the current model with `3`

entities (ODEs) and `3`

parameters, a total of `9`

sensitivities are calculated at each output time, i.e. `y1`

w.r.t `p1`

, `p2`

, `p3`

, `y2`

w.r.t. `p1`

, `p2`

, `p3`

and so on. The first 3 (`length(y)`

) columns give sensitivity w.r.t the first parameter, the next 3 (`length(y)`

) columns give sensitivity w.r.t the second parameter and so on.

In the Sensitivity Matrix output for the systems of equations described above, the first column gives output time, the next `3`

columns provide sensitivity of `y1`

, `y2`

and `y3`

w.r.t first parameter (say `p1`

), the next three columns provide sensitivity of `y1`

, `y2`

and `y3`

w.r.t. the second parameter (`p2`

) and so on. The output Sensitivity Matrix is given below. The sensitivity values match with the values provided in the `CVODES`

documentation.

```
> df1
[,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
[1,] 0e+00 0.000000e+00 0.000000e+00 0.000000e+00 0.000000e+00 0.000000e+00 0.000000e+00 0.000000e+00 0.000000e+00 0.000000e+00
[2,] 4e-01 -3.561085e-01 3.902252e-04 3.557183e-01 9.483149e-08 -2.132509e-10 -9.461823e-08 -1.573297e-11 -5.289692e-13 1.626194e-11
[3,] 4e+00 -1.876130e+00 1.792229e-04 1.875951e+00 2.961233e-06 -5.830758e-10 -2.960650e-06 -4.932970e-10 -2.762408e-13 4.935732e-10
[4,] 4e+01 -4.247395e+00 4.592812e-05 4.247349e+00 1.372964e-05 -2.357270e-10 -1.372941e-05 -2.288274e-09 -1.138015e-13 2.288387e-09
[5,] 4e+02 -5.958192e+00 3.545986e-06 5.958189e+00 2.273754e-05 -2.260807e-11 -2.273752e-05 -3.789554e-09 -4.994795e-14 3.789604e-09
[6,] 4e+03 -4.750132e+00 -5.991971e-06 4.750138e+00 1.880937e-05 2.312156e-11 -1.880939e-05 -3.134824e-09 -1.875976e-14 3.134843e-09
[7,] 4e+04 -1.574902e+00 -2.761679e-06 1.574905e+00 6.288404e-06 1.100645e-11 -6.288415e-06 -1.047876e-09 -4.536508e-15 1.047881e-09
[8,] 4e+05 -2.363168e-01 -4.584043e-07 2.363173e-01 9.450741e-07 1.832930e-12 -9.450760e-07 -1.574929e-10 -6.362045e-16 1.574935e-10
[9,] 4e+06 -2.566355e-02 -5.105587e-08 2.566361e-02 1.026491e-07 2.042044e-13 -1.026493e-07 -1.711080e-11 -6.851356e-17 1.711087e-11
[10,] 4e+07 -2.597859e-03 -5.190342e-09 2.597864e-03 1.039134e-08 2.076100e-14 -1.039136e-08 -1.732552e-12 -6.930923e-18 1.732559e-12
[11,] 4e+08 -2.601996e-04 -5.199259e-10 2.602002e-04 1.040802e-09 2.079717e-15 -1.040804e-09 -1.737821e-13 -6.951356e-19 1.737828e-13
[12,] 4e+09 -2.648142e-05 -5.616896e-11 2.648147e-05 1.059193e-10 2.246502e-16 -1.059195e-10 -1.804535e-14 -7.218146e-20 1.804542e-14
[13,] 4e+10 -2.899376e-06 -7.759920e-12 2.899383e-06 1.159764e-11 3.104024e-17 -1.159768e-11 -1.727574e-15 -6.910296e-21 1.727581e-15
```

In future, I intend to provide options to select specific entities and parameters with respect to which sensitivities are to be computed as the sensitivity matrix can get very large for medium to large models.

The package `sundialr`

provides a way to interface with the famous `SUNDIALS`

C library to solver initial value problems. The package allows the system of differential equations to be written in `R`

or using `Rcpp`

. Currently only a single initialization of the ODE is supported, but we plan to add repeated initializations (e.g., repeated dosing) in near future for `CVODE`

. For sensitivities, currently, calculation of forward sensitivities for all entities with respect to all the parameters in the model is implemented. However the ability to select specific entities and parameters for which sensitivities is to be calculated will be added soon. As a note, since this package is under active development, the interfaces of both `CVODE`

and `CVODES`

(i.e., the function signatures) may change in the future versions. Please keep this mind if you intend to use `sundialr`

in your applications. In near future, interface for other solvers from the `C`

library such as`IDA/IDAS`

and `ARKODE`

will also be added.