Levi Ben Gershon (1288-1344) (see also here) is commonly known to us as the RaLBa"G. Again, this is a nickname rather than a pseudonym- RLBG = "Rabbi Levi Ben Gershon", much in the same way as Shah Rikh Khan is known as SRK.
He wrote three mathematics books including Maaseh Hoshev, which "... is notable for its early use of proof by mathematical induction, and pioneering work in combinatorics. "