I am looking for a book I read (I think late 70's early 80's). It's about a young math wiz who travels back in time to the Middle Ages using math formulas. He is talented at fencing and uses his fencing ability to defeat knights using much more cumbersome swords. I don't remember much more. Thanks for any help
"The Mathematics of Magic" by L. Sprague de Camp and Fletcher Pratt, published in the August 1940 edition of Unknown magazine, which can be read online at UNZ.org.
It is based around the infinity of possible universes, and the rules governing each of them. For example, just like the law of gravitation, there are laws in magical universes such as the law of similarity (when working a spell, something that is similar to the object being enchanted is used in the spell).
This happens when the protagonist, Shea, tries to create a unicorn with a model of a horse-like figure with one horn, and instead ends up with
a friendly forest rhinoceros
A group of psychologists believe that identifying and reading out the laws of this universe will propel them into another universe. Travel between these universes is possible, however it has never been done before. People have seen these alternate universes, and written stories about them, but been unable to travel to them. (Any universe created by any author is actually real, and can be travelled to: for example, in this story they travel to the universe seen by Edmund Spenser in his poem "The Faerie Queene".
Shea, the younger fencing expert, and Chalmers, the older man who becomes an adept at magic, work together to destroy the Enchanter's Guild which is cursing Queen Gloriana's knights.
Below follows a summary of your key points of the story, and how this story matches:
It's about a young math wiz who travels back in time to the Middle Ages using math formulas
Chalmers brushed the mop of gray hair from his eyes and looked pleased. "Amazing! Of course, I held that the transfer of the physical body to another spacetime frame by symbolic logic was possible or I should not have said so. But it is always a shock to have so... uh... farfetched a deduction confirmed by experimental proof.
"You know enough mathematics to be aware that the 'fourth dimension', so called, is only a dimension in the sense of a measurable quality, like color or density. The same applies for the interuniversal dimensions. I maintain—"
He is talented at fencing and uses his fencing ability to defeat knights using much more cumbersome swords
The antique shops had nothing better; their antique weapons were mostly Civil War cavalry sabers. Shea decided to use his own fencing épée. It had a rather stiff blade, and if he unscrewed the point d'arret, ground the end down to a sharp point, and contrived some kind of sheath, the weapon would do till he got something better.
He [the knight Sir Hardimour] checked, and held his sword out in front of him, trying to imitate Shea's fencing position.
That explained a lot, thought Shea: the actions of the two knights, for one thing, and for another, why the magicians were so polite to him, though his rating was no more than that of an apprentice. There would be something practically supernatural about modern fencing technique to these people.