# Steady State Heat Equation - No Time Dependance - NonlinearProblem

Sometimes it is desirable to solve an equation that has no time evolution, such as the steady state heat equation:

using ModelingToolkit, MethodOfLines, DomainSets, NonlinearSolve

@parameters x y
@variables u(..)
Dxx = Differential(x)^2
Dyy = Differential(y)^2

eq = Dxx(u(x, y)) + Dyy(u(x, y)) ~ 0

bcs = [u(0, y) ~ x * y,
u(1, y) ~ x * y,
u(x, 0) ~ x * y,
u(x, 1) ~ x * y]

# Space and time domains
domains = [x ∈ Interval(0.0, 1.0),
y ∈ Interval(0.0, 1.0)]

@named pdesys = PDESystem([eq], bcs, domains, [x, y], [u(x, y)])

dx = 0.1
dy = 0.1

# Note that we pass in nothing for the time variable t here since we
# are creating a stationary problem without a dependence on time, only space.
discretization = MOLFiniteDifference([x => dx, y => dy], nothing, approx_order=2)

prob = discretize(pdesys, discretization)
sol = NonlinearSolve.solve(prob, NewtonRaphson())

grid = get_discrete(pdesys, discretization)

u_sol = map(d -> sol[d], grid[u(x, y)])

using Plots

heatmap(grid[x], grid[y], u_sol, xlabel="x values", ylabel="y values",
title="Steady State Heat Equation")