Fierce soldiers known as?

Answer 1

Samurai? Japan, WW2

Amazons? South America

Marines? Spec Ops? Airborne Rangers? of any nation

Women's Brigade of Death (WW1)?

Battalion of Death? Spain's Civil War

Vikings?

Mongolian Hoardes under Ghengis Kahn?

Attila the Hun's troops

Alexander the Great's Greek troops

Stonewall Jackson's troops Civil War

George Patton's Army WW2

Either side in the Russia/Ukraine war as both sides fight to the death

Gen. MacArthur's troops WW2

Robert L. Howard's Special Forces (Vietnam)

Audie Murphy WW2

Benedict Arnold and his troops winning the Battle of Saratoga while vastly outnumbered by the British

BUT you're looking for a geometry/mathematician who was a fierce warrior? That might be General James A. Garfield, later President Garfield, youngest general in the Civil War and who also came up with a new geometric proof of the Pythagorian Theorem. Not to be confused with the comic strip cat of same name.