# Origin of notation "R with a stroke on the leg" for the square-root (℞)

The following text from Ars magna (1545) by Girolamo Cardano is known as the inception of complex numbers: "imaginaberis ℞ m 15" (You will imagine the square root of minus 15):

The "R" with the stroke on its leg could read as ℞ (Rx), an abbreviation for radix, or root in Latin. Was this notation frequent, and does it have a known origin?

Note: this appears on the page Cap. XXXVII. De Regula fals. 287, Demonstratio of this Latin version of Artis magnae sive de regulis algebraicis, liber unus.

• Leo Corry, in his Brief History of Numbers, refers to ℞ as "Cardano's shorthand", citing Gavagna 2012, page 7. Corry continues by noting that Bombelli used the unadorned "R", e.g., "R.c" for cubed root. This suggests that perhaps the use of ℞ in mathematics was introduced by Cardano himself. The use of ℞ in medical prescriptions continues to this day and we know that medieval physicians used ℞ so perhaps (it's a stretch) Bombelli and others wished to avoid possible confusion.
– nwr
Nov 22 '20 at 18:48
• Thank you for this source. I just read from Cadori (A history of mathematical notations) that the use of Rx as radix is also seen in Leonardo of Pisa (1170-1240) and Luca Pacioli (1447-1517). I will dig a bit further. Nov 22 '20 at 20:41
• Leonardo and Luca used ℞ ambiguously, for the unknown and the square root, the former use is older. For uses contemporaneous with Cardano, e.g. by Peletier, see Printers and algebraists in mid-16th century France by Loget. Nov 23 '20 at 10:54
• @Nick but the medical "Rx" is simply an abbreviation. "Dx" for diagnosis and "Rx" for recommendation/ prescription. Nov 23 '20 at 13:03
• Abbreviation for "take" (recipe) if I read it well Nov 23 '20 at 13:15