Last week we released the first public version of `dde`

. This package implements a method for solving delay differential equations, which we use with `odin`

to model disease dynamics.

With ordinary differential equations, you express the system of equations as *dy/dt = f(y(t), t)*; the rate of change of the system depends on the current state of the system and the current time, but with delay differential equations *dy/dt* also depends on *y(t - τ)*, where *τ* is a length of time back into the past. In general these are hard to solve numerically but there is a large class of useful equations with constant delays that are both interesting and tractable.

Integrating delay differential equations allows researchers in our department to model relationships where (say) the number of mosquitos entering a lifecycle phase now depends on the number of people who were bitten several days ago.

`dde`

implements the method of Hairer, Norsett and Wanner (1993) where we use an ODE solver that can accurately interpolate to points within steps that it takes along with a ring buffer to store the history. It only works with non-stiff systems but we have found it to work well on large systems of equations where the DDE support in `deSolve`

(implemented using `lsoda`

) stopped working.

`dde`

is now available on CRAN and can be installed with

```
install.packages("dde")
```

To get started see the package vignette.