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.