Softened 2D gravity solved with an adaptive quadtree and Cartesian multipole / local expansions. Far-field forces travel through the tree; only neighbouring leaves are summed directly.
M2M → M2L → L2L → P2P| Particles | — |
| FPS | — |
| FMM solve | — |
| Tree nodes | — |
| M2L interactions | — |
| Sample force error | — |