zanzibar: Yes, <perfidious>, I agree that different transliterations could be used.
Sometimes, there's a little more to the story, e.g. some cultural wrinkle somebody like myself might not know about.
Other times there might be little more to the story. I'd like to see how BCM or some other English-language contemporaneous reporting spelled his name.
In Cyrillic it's this:
<Александр Сергеевич Сергеев>
which google translates as <Alexander Sergeevich Sergeev>.
My natural bias is to follow RUSbase as much as possible (and also to use the simpler spellings if possible). Happens to match Google translate.
I'll just note that 365chess also uses Sergeev.
Wiki, however, uses <Aleksandr Sergeyevich Sergeyev>.
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Mato has a video of Sozin demolishing Serge(i)ev's K-side:
from this game:
A Sergeiev vs V Sozin, 1924