Sooo...
http://wiki.pokemonspeedruns.com/index.php?title=Pokémon_Red/Blue_Wild_DVs
Didn't see this mentioned here yet, but it turns out that thanks to Generation I's RNG and how random wild encounters are handled, only a small fraction (~10%) of the 65,536 theoretical sets of DVs are actually ever possible. Surprise, surprise, 15/15/15/15 isn't one of them. This is thanks to consecutive RNG outputs being highly correlated rather than independent, and the fact that the RNG has to land on an encounter in the first place (e.g. 25/256 chance for each step you take on Route 1) before it can generate any DVs. As the linked article describes, the more frequent wild encounters are in an area, the larger the range of possible DV spreads for those wild Pokemon, and vice versa.
This is theoretically bad news for some OU Pokemon like Golem and Gengar, as the only way to obtain one is in the wild in an area(s) with particularly sparse wild encounters, namely caves (which naturally have a lower encounter rate) and Pokemon Tower (which also has a low encounter rate). In both of these areas, less than 6% of the theoretical DV spreads are actually physically possible for the RNG to generate, and they're a (proper) subset of any areas with a higher encounter rate.
If I understand correctly, anything that isn't a random wild encounter (so gifts, Game Corner, Snorlax, legendaries, in-game trades, etc.) is not subject to this limitation, as there are no constraints on what the RNG can be immediately before the DVs are generated. This would also apply to, say, the Mew glitch, which generates a stationary encounter. I don't know if GSC's RNG works the same way, but I'm assuming tradebacks aren't being considered here.
tl;dr: any 15/15/15/15 Golem or Gengar in pure RBY is illegal
http://wiki.pokemonspeedruns.com/index.php?title=Pokémon_Red/Blue_Wild_DVs
Didn't see this mentioned here yet, but it turns out that thanks to Generation I's RNG and how random wild encounters are handled, only a small fraction (~10%) of the 65,536 theoretical sets of DVs are actually ever possible. Surprise, surprise, 15/15/15/15 isn't one of them. This is thanks to consecutive RNG outputs being highly correlated rather than independent, and the fact that the RNG has to land on an encounter in the first place (e.g. 25/256 chance for each step you take on Route 1) before it can generate any DVs. As the linked article describes, the more frequent wild encounters are in an area, the larger the range of possible DV spreads for those wild Pokemon, and vice versa.
This is theoretically bad news for some OU Pokemon like Golem and Gengar, as the only way to obtain one is in the wild in an area(s) with particularly sparse wild encounters, namely caves (which naturally have a lower encounter rate) and Pokemon Tower (which also has a low encounter rate). In both of these areas, less than 6% of the theoretical DV spreads are actually physically possible for the RNG to generate, and they're a (proper) subset of any areas with a higher encounter rate.
If I understand correctly, anything that isn't a random wild encounter (so gifts, Game Corner, Snorlax, legendaries, in-game trades, etc.) is not subject to this limitation, as there are no constraints on what the RNG can be immediately before the DVs are generated. This would also apply to, say, the Mew glitch, which generates a stationary encounter. I don't know if GSC's RNG works the same way, but I'm assuming tradebacks aren't being considered here.
tl;dr: any 15/15/15/15 Golem or Gengar in pure RBY is illegal
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