I’m trying to solve an elliptic differential equation that has multiple derivative terms and wanted to use DiffEqOperators.jl to build the matrices. Is it possible to sum DiffEqOperators and then solve using backslash? When I try this, I get an error. Here’s an example, based on https://discourse.ju…

u = SparseMatrixCSC((0.5*Δ+0.5*Δ)*bc) \ f Hmm that’s missing a concretization. Can you open an issue?

I’m not sure what the solution is, but FWIW, this happens because the ghost operators have their own backslash defined here , which I think is necessary since the ghost operator is not strictly a linear operator but an affine one AFAIU. Maybe you need to add a *(operator combination, bc), similar to…

Yeah exactly. It seems like it’s just a missing method.

This does work: A1, b1 = sparse(0.5*Δ*bc) A2, b2 = sparse(0.5*Δ*bc) As = A1 + A2 u = As \ f so I can combine linear operators this way, though it’s less elegant than adding a method for DiffEqOperatorCombination as @briochemc suggested. Separately but related, calling sparse on a DiffEqOperator i…

[image] bclyons12: I would have thought sparse on a DerivativeOperator or GhostDerivativeOperator would have done something like my by_hand function. Any thoughts? At it to the issue. It should be doing it like you did by hand, it may be falling back to an iteration on the indices method.

I opened a separate issue on git https://github.com/SciML/DiffEqOperators.jl/issues/392

Solving linear system with summed DiffEqOperators

General Usage

bclyons12 May 13, 2021, 1:49am 5

Submitted as
https://github.com/SciML/DiffEqOperators.jl/issues/391

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Solving linear system with summed DiffEqOperators

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