Interesting; I was taught it was "Chebychev", which as second ranking
doesn't even come close to "Chebyshev"...
"Chebychev" is downright bizarre. The first "Ch" is as in English "Chebyshev", the second is as in French "Tchebychev". The other variants are mostly explicable: for example, in German it's "Tschebyschew".
The final vowel is written "e" but pronounced "o" in Russian, and can come out either way in transliteration. Likewise, the final "v" is pronounced "f", and can be written "v" or "ff" in transliteration. Tolstoy was affected by this too: his first name is usually given as either "Leo" (which is what it means) or "Lev" nowadays, but in the 19C it was usually spelled "Lyoff" (one syllable), which is how it's pronounced.