In older books, I've noticed that authors tended to use $J$ to denote (usually) the natural numbers and (less commonly) the integers. Does anyone have any idea why that might've been? A few examples include:
- Rudin's Principles of Mathematical Analysis
- Herstein's Topics in Algebra
- McCarthy's Algebraic Extensions of Fields
Earliest Uses of Symbols of Number Theory mentions: "The second edition of Birkhoff and MacLane, Survey of Modern Algebra (1953) ... has J for integers." This answer to a secondary student's question says that is the earliest reference she can find and speculated the same thing as @Gerry.
Sorry, only answering because I don't have the reputation points yet to comment.