Time-dependent events in ODE

Specific Domains Modelling & Simulations
4

You need to split into two callbacks (as your code errors on this), see http://docs.juliadiffeq.org/latest/features/callback_functions.html#CallbackSet-1 on how to combine them. Then the other error is

function affect!(integrator)
    integrator.u[1] = stim
end

as stated in the docs: http://docs.juliadiffeq.org/latest/features/callback_functions.html#Example-1:-Bouncing-Ball-1

So all in all, slightly modified:

using Plots
using DifferentialEquations

# Parameters
const k21 = 0.14*24
const k12 = 0.06*24
const ke = 1.14*24
const α = 0.5
const β = 0.05
const η = 0.477
const μ = 0.218
const k1 = 0.5
const V1 = 6

# Time
maxtime = 0.1
tspan = (0.0, maxtime)

# Dose
const stim = 100

# Initial conditions
x0 = [0 0 2e11 8e11]

# Model equations
function system(dy, y, p, t)
  dy[1] = k21*y[2] - (k12 + ke)*y[1]
  dy[2] = k12*y[1] - k21*y[2]
  dy[3] = (α - μ - η)*y[3] + β*y[4] - k1/V1*y[1]*y[3]
  dy[4] = μ*y[3] - β*y[4]
end

# Events
function condition(y, t, integrator)
    t - 0.05
end
function affect!(integrator)
    integrator.u[1] = stim
end
cb = ContinuousCallback(condition, affect!)

# Solve
prob = ODEProblem(system, x0, tspan)
sol = solve(prob, Rodas4(), callback = cb)

# Plotting
plot(sol, layout = (2, 2))

(But please, next time provide an example which is more minimal and runs faster.)

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