We produce a nonpositively curved square complex X containing exactly four squares. Its universal cover X̃≅T4 × T4 is isomorphic to the product of two 4–valent trees. The group π1X is a lattice in Aut(X̃) but π1X is not virtually a nontrivial product of free groups. There is no such example with fewer than four squares. The main ingredient in our analysis is that X̃ contains an “anti-torus” which is a certain aperiodically tiled plane.
Janzen, David  1 ; Wise, Daniel T  1
@article{10_2140_agt_2009_9_2191,
author = {Janzen, David and Wise, Daniel~T},
title = {A smallest irreducible lattice in the product of trees},
journal = {Algebraic and Geometric Topology},
pages = {2191--2201},
year = {2009},
volume = {9},
number = {4},
doi = {10.2140/agt.2009.9.2191},
url = {http://geodesic.mathdoc.fr/articles/10.2140/agt.2009.9.2191/}
}
TY - JOUR AU - Janzen, David AU - Wise, Daniel T TI - A smallest irreducible lattice in the product of trees JO - Algebraic and Geometric Topology PY - 2009 SP - 2191 EP - 2201 VL - 9 IS - 4 UR - http://geodesic.mathdoc.fr/articles/10.2140/agt.2009.9.2191/ DO - 10.2140/agt.2009.9.2191 ID - 10_2140_agt_2009_9_2191 ER -
Janzen, David; Wise, Daniel T. A smallest irreducible lattice in the product of trees. Algebraic and Geometric Topology, Tome 9 (2009) no. 4, pp. 2191-2201. doi: 10.2140/agt.2009.9.2191
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