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):

Ars magna, complex numbers inception

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.

Artis magnae sive de regulis algebraicis, liber unus, Cap. XXXVII. De Regula fals. 287, Demonstratio

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    $\begingroup$ 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. $\endgroup$
    – nwr
    Nov 22 '20 at 18:48
  • $\begingroup$ 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. $\endgroup$ Nov 22 '20 at 20:41
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    $\begingroup$ 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. $\endgroup$
    – Conifold
    Nov 23 '20 at 10:54
  • $\begingroup$ @Nick but the medical "Rx" is simply an abbreviation. "Dx" for diagnosis and "Rx" for recommendation/ prescription. $\endgroup$ Nov 23 '20 at 13:03
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    $\begingroup$ Abbreviation for "take" (recipe) if I read it well $\endgroup$ Nov 23 '20 at 13:15

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