User talk:MWinter4/4-flat graphs

Future work

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4-flat graphs are closed under cloning edges and $\Delta \mathrm {Y}$ -transformations. It is conjectured that they are also closed under $\mathrm {Y} \Delta$ -transformations.^{[citation needed]}
Alternative to the notion of 4-flat graphs based on almost embeddings ( ${\mathcal {I}}_{4}$ ) with its higher-dimensional analogues ( ${\mathcal {I}}_{d}$ ) and leading to van der Holst's topological graph invariant $\sigma$ .
Alternative definitions of 4-flat using general CW-complex (instead of regular CW-complexes).^{[citation needed]}