# Optimization-Based ODE Parameter Estimation

We choose to optimize the parameters on the Lotka-Volterra equation. We do so by defining the function as a function with parameters:

function f(du,u,p,t)
du[1] = dx = p[1]*u[1] - u[1]*u[2]
du[2] = dy = -3*u[2] + u[1]*u[2]
end

u0 = [1.0;1.0]
tspan = (0.0,10.0)
p = [1.5]
prob = ODEProblem(f,u0,tspan,p)

We create data using the numerical result with a=1.5:

sol = solve(prob,Tsit5())
t = collect(range(0,stop=10,length=200))
using RecursiveArrayTools # for VectorOfArray
randomized = VectorOfArray([(sol(t[i]) + .01randn(2)) for i in 1:length(t)])
data = convert(Array,randomized)

Here we used VectorOfArray from RecursiveArrayTools.jl to turn the result of an ODE into a matrix.

If we plot the solution with the parameter at a=1.42, we get the following:

Notice that after one period this solution begins to drift very far off: this problem is sensitive to the choice of a.

To build the objective function for Optim.jl, we simply call the build_loss_objective function:

cost_function = build_loss_objective(prob,Tsit5(),L2Loss(t,data),
maxiters=10000,verbose=false)

This objective function internally is calling the ODE solver to get solutions to test against the data. The keyword arguments are passed directly to the solver. Note that we set maxiters in a way that causes the differential equation solvers to error more quickly when in bad regions of the parameter space, speeding up the process. If the integrator stops early (due to divergence), then those parameters are given an infinite loss, and thus this is a quick way to avoid bad parameters. We set verbose=false because this divergence can get noisy.

Before optimizing, let's visualize our cost function by plotting it for a range of parameter values:

vals = 0.0:0.1:10.0
using Plots; plotly()
plot(vals,[cost_function(i) for i in vals],yscale=:log10,
xaxis = "Parameter", yaxis = "Cost", title = "1-Parameter Cost Function",
lw = 3)

Here we see that there is a very well-defined minimum in our cost function at the real parameter (because this is where the solution almost exactly fits the dataset).

Now this cost function can be used with Optim.jl in order to get the parameters. For example, we can use Brent's algorithm to search for the best solution on the interval [0,10] by:

using Optim
result = optimize(cost_function, 0.0, 10.0)

This returns result.minimizer[1]==1.5 as the best parameter to match the data. When we plot the fitted equation on the data, we receive the following:

Thus we see that after fitting, the lines match up with the generated data and receive the right parameter value.

We can also use the multivariate optimization functions. For example, we can use the BFGS algorithm to optimize the parameter starting at a=1.42 using:

result = optimize(cost_function, [1.42], BFGS())

Note that some of the algorithms may be sensitive to the initial condition. For more details on using Optim.jl, see the documentation for Optim.jl. We can improve our solution by noting that the Lotka-Volterra equation requires that the parameters are positive. Thus following the Optim.jl documentation we can add box constraints to ensure the optimizer only checks between 0.0 and 3.0 which improves the efficiency of our algorithm:

lower = [0.0]
upper = [3.0]
result = optimize(cost_function, lower, upper, [1.42], Fminbox(BFGS()))

Lastly, we can use the same tools to estimate multiple parameters simultaneously. Let's use the Lotka-Volterra equation with all parameters free:

function f2(du,u,p,t)
du[1] = dx = p[1]*u[1] - p[2]*u[1]*u[2]
du[2] = dy = -p[3]*u[2] + p[4]*u[1]*u[2]
end

u0 = [1.0;1.0]
tspan = (0.0,10.0)
p = [1.5,1.0,3.0,1.0]
prob = ODEProblem(f2,u0,tspan,p)

We can build an objective function and solve the multiple parameter version just as before:

cost_function = build_loss_objective(prob,Tsit5(),L2Loss(t,data),
maxiters=10000,verbose=false)
result_bfgs = Optim.optimize(cost_function, [1.3,0.8,2.8,1.2], Optim.BFGS())

We can also use First-Differences in L2Loss by passing the kwarg differ_weight which decides the contribution of the differencing loss to the total loss.

cost_function = build_loss_objective(prob,Tsit5(),L2Loss(t,data,differ_weight=0.3,data_weight=0.7),
maxiters=10000,verbose=false)
result_bfgs = Optim.optimize(cost_function, [1.3,0.8,2.8,1.2], Optim.BFGS())

To solve it using LeastSquaresOptim.jl, we use the build_lsoptim_objective function:

cost_function = build_lsoptim_objective(prob1,t,data,Tsit5())

The result is a cost function which can be used with LeastSquaresOptim. For more details, consult the documentation for LeastSquaresOptim.jl:

using LeastSquaresOptim # for LeastSquaresProblem
x = [1.3,0.8,2.8,1.2]
res = optimize!(LeastSquaresProblem(x = x, f! = cost_function,
output_length = length(t)*length(prob.u0)),
LeastSquaresOptim.Dogleg(LeastSquaresOptim.LSMR()))

We can see the results are:

println(res.minimizer)

Results of Optimization Algorithm
* Algorithm: Dogleg
* Minimizer: [1.4995074428834114,0.9996531871795851,3.001556360700904,1.0006272074128821]
* Sum of squares at Minimum: 0.035730
* Iterations: 63
* Convergence: true
* |x - x'| < 1.0e-15: true
* |f(x) - f(x')| / |f(x)| < 1.0e-14: false
* |g(x)| < 1.0e-14: false
* Function Calls: 64
* Multiplication Calls: 135

and thus this algorithm was able to correctly identify all four parameters.

We can also use Multiple Shooting method by creating a multiple_shooting_objective

function ms_f(du,u,p,t)
dx = p[1]*u[1] - p[2]*u[1]*u[2]
dy = -3*u[2] + u[1]*u[2]
end
ms_u0 = [1.0;1.0]
tspan = (0.0,10.0)
ms_p = [1.5,1.0]
ms_prob = ODEProblem(ms_f,ms_u0,tspan,ms_p)
t = collect(range(0,stop=10,length=200))
data = Array(solve(ms_prob,Tsit5(),saveat=t,abstol=1e-12,reltol=1e-12))
bound = Tuple{Float64, Float64}[(0, 10),(0, 10),(0, 10),(0, 10),
(0, 10),(0, 10),(0, 10),(0, 10),
(0, 10),(0, 10),(0, 10),(0, 10),
(0, 10),(0, 10),(0, 10),(0, 10),(0, 10),(0, 10)]

ms_obj = multiple_shooting_objective(ms_prob,Tsit5(),L2Loss(t,data);discontinuity_weight=1.0,abstol=1e-12,reltol=1e-12)

This creates the objective function that can be passed to an optimizer from which we can then get the parameter values and the initial values of the short time periods keeping in mind the indexing.

# ]add BlackBoxOptim
using BlackBoxOptim

result = bboptimize(ms_obj;SearchRange = bound, MaxSteps = 21e3)
result.archive_output.best_candidate[end-1:end]

Giving us the results as

Starting optimization with optimizer BlackBoxOptim.DiffEvoOpt{BlackBoxOptim.FitPopulation{Float64},BlackBoxOptim.RadiusLimitedSelector,BlackBoxOptim.AdaptiveDiffEvoRandBin{3},BlackBoxOptim.RandomBound{BlackBoxOptim.RangePerDimSearchSpace}}

Optimization stopped after 21001 steps and 136.60030698776245 seconds
Termination reason: Max number of steps (21000) reached
Steps per second = 153.7405036862868
Function evals per second = 154.43596332393247
Improvements/step = 0.17552380952380953
Total function evaluations = 21096

Best candidate found: [0.997396, 1.04664, 3.77834, 0.275823, 2.14966, 4.33106, 1.43777, 0.468442, 6.22221, 0.673358, 0.970036, 2.05182, 2.4216, 0.274394, 5.64131, 3.38132, 1.52826, 1.01721]

Fitness: 0.126884213

Out[4]:2-element Array{Float64,1}:
1.52826
1.01721

Here as our model had 2 parameters, we look at the last two indexes of result to get our parameter values and the rest of the values are the initial values of the shorter timespans as described in the reference section.

The objective function for Two Stage method can be created and passed to an optimizer as

two_stage_obj = two_stage_method(ms_prob,t,data)
result = Optim.optimize(two_stage_obj, [1.3,0.8,2.8,1.2], Optim.BFGS()
)
Results of Optimization Algorithm
* Algorithm: BFGS
* Starting Point: [1.3,0.8,2.8,1.2]
* Minimizer: [1.5035938533664717,0.9925731153746833, ...]
* Minimum: 1.513400e+00
* Iterations: 9
* Convergence: true
* |x - x'| ≤ 0.0e+00: false
|x - x'| = 4.58e-10
* |f(x) - f(x')| ≤ 0.0e+00 |f(x)|: false
|f(x) - f(x')| = 5.87e-16 |f(x)|
* |g(x)| ≤ 1.0e-08: true
|g(x)| = 7.32e-11
* Stopped by an increasing objective: false
* Reached Maximum Number of Iterations: false
* Objective Calls: 31
* Gradient Calls: 31

The default kernel used in the method is Epanechnikov others that are available are Uniform, Triangular, Quartic, Triweight, Tricube, Gaussian, Cosine, Logistic and Sigmoid, this can be passed by the kernel keyword argument. loss_func keyword argument can be used to pass the loss function (cost function) you want to use and mpg_autodiff enables Auto Differentiation.