ᐜ The function SSCG(n) is the function, devised by the goodly & mickle-renownèd Harvey Friedman , that's the maximum length of a sequence of simple (ie without loops or multiple edges) subcubic (ie in which every vertex has valency @most 3) graphs
G₁ , G₂ , G₃ ...
in which Gₖ has @most n+k vertices, & for no h<k is Gₕ a minor of Gₖ . This sequence is renowned for yielding transcendentally large numbers. It's somewhat like the TREE() sequence, but (apparently) yields larger №s.
But thesedays there's a veritable menagerie of weïrd systems that have been devised, entailing all-manner of lexicographical manipulations, & wot-not (eg Fish №s, & Loader's №s) that yield №s fabulously greater still .
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From
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Complex Projective 4-Space – Graph Minors
https://cp4space.hatsya.com/2013/01/13/graph-minors/
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Note that there is no requirement that the graphs in question be connected. In the diagrams below, I’ll indicate the disjoint union of graphs with a plus (+) symbol, and the disjoint union of n identical graphs by preceding it with a number n. Firstly, the best lower bound I can attain for SSCG(2) is merely 3 × 2^(3 × 2^95) − 9 (i.e. less than a googolplex), which I conjecture is optimal.
It’s not difficult to show that the first few moves must be optimal. After the first move, we’re restricted to forests, and the second move further restricts us to disjoint unions of line graphs. I proceeded using a greedy algorithm, ensuring that the corresponding ordinal (replace each line graph on n vertices with the ordinal ω^(n−1)) is as large as possible. Greedy algorithms often give optimal solutions to simple problems, although others (travelling salesman, for example) resist this approach.
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