**Reference**:- Erik D. Demaine, Maarten Löffler, and Christiane Schmidt, “Rectangular Spiral Galaxies are still hard”,
*Computational Geometry: Theory and Applications*, volume 110, 2023, pages 101949. **Abstract**:-
Spiral Galaxies is a pencil-and-paper puzzle played on a grid of unit squares:
given a set of points called
*centers*, the goal is to partition the grid into polyominoes such that each polyomino contains exactly one center and is 180° rotationally symmetric about its center. We show that this puzzle is NP-complete, ASP-complete, and #P-complete even if (a) all solutions to the puzzle have rectangles for polyominoes; or (b) the polyominoes are required to be rectangles and all solutions to the puzzle have just 1 × 1, 1 × 3, and 3 × 1 rectangles. The proof for the latter variant also implies NP/ASP/#P-completeness of finding a noncrossing perfect matching in distance-2 grid graphs where edges connect vertices of Euclidean distance 2. Moreover, we prove NP-completeness of the design problem of minimizing the number of centers such that there exists a set of galaxies that exactly cover a given shape. **Comments**:- This paper is also available from ScienceDirect.
**Length**:- The paper is 31 pages.
**Availability**:- The paper is available in PDF (1040k).
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**Related papers**:- SpiralGalaxies_MOVES2017 (Spiral Galaxies Font)

See also other papers by Erik Demaine.

Last updated July 23, 2024 by Erik Demaine.