A differential-algebraic equation is defined by an implicit function
`f(du,u,p,t)=0`

. All of the controls are the same as the
other examples, except here you define a function which returns the
residuals for each part of the equation to define the DAE. The initial
value `u0`

and the initial derivative `du0`

are
required, though they do not necessarily have to satisfy `f`

(known as inconsistent initial conditions). The methods will
automatically find consistent initial conditions. In order for this to
occur, `differential_vars`

must be set. This vector states
which of the variables are differential (have a derivative term), with
`false`

meaning that the variable is purely algebraic.

This example shows how to solve the Robertson equation:

```
f <- function (du,u,p,t) {
resid1 = - 0.04*u[1] + 1e4*u[2]*u[3] - du[1]
resid2 = + 0.04*u[1] - 3e7*u[2]^2 - 1e4*u[2]*u[3] - du[2]
resid3 = u[1] + u[2] + u[3] - 1.0
c(resid1,resid2,resid3)
}
u0 <- c(1.0, 0, 0)
du0 <- c(-0.04, 0.04, 0.0)
tspan <- c(0.0,100000.0)
differential_vars <- c(TRUE,TRUE,FALSE)
prob <- de$DAEProblem(f,du0,u0,tspan,differential_vars=differential_vars)
sol <- de$solve(prob)
udf <- as.data.frame(t(sapply(sol$u,identity)))
plotly::plot_ly(udf, x = sol$t, y = ~V1, type = 'scatter', mode = 'lines') %>%
plotly::add_trace(y = ~V2) %>%
plotly::add_trace(y = ~V3)
```