REGARDING RODERER: A Novel About Genius by Guillermo Martinez. (Wyatt/St. Martin's: $13.95; 96 pp.) As a novelist, Martinez is a damn fine mathematician (a Ph.D., as a matter of fact, now at Oxford). A pretty fair philosopher, too. An exegete he's not. Granted, he's set himself a monumental task in a minimal space: A novel about pure thought. Gustavo Roderer, the young Argentine of the title, is a genius. His quest: nothing less than true knowledge, "not the quota of tolerable wisdom, but . . . the Logos that the devil and God safeguard together," the ultimate formula/philosophy that will explain all things. Martinez, when he's not tossing about theses on some of history's more rarefied intellectuals, makes the acquisition of such knowledge seem frightening, dangerous. Surely to Roderer it is: He thinks himself to death. The narrator himself is extremely bright--he has to be--but with the sort of intelligence that turns outward. Roderer, his high school classmate, turns his brain--his soul--inward. He has long since absorbed the weightiest of books and spends his time in solitary contemplation. (He does a little opium, too, but what the hell.) Does Roderer find the truth? He does. Conveniently for Martinez, he dies before he can explain it. Dies, mind you, with these words to the Creator: "Let me in, I am the first." No argument here.