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anton@lemmy.blahaj.zone 3 weeks agoI know how how natural numbers work, but the axioms in the comment i replied to are not enough to define them.
Not sure what you mean by ‘loops’
There could be a number n such that m=s(n) and n=s(m).
This would be precluded by taking the axiom of induction or the trichotomy axiom.
If we only take the latter we can still make a second number line, that runs “parallel” to the “propper number line” like:
n,s(n),s(s(n)),s(s(s(n))),... 0,s(0),s(s(0)),s(s(s(0))),...
there are no natural numbers that are negative
I know, but the given axioms don’t preclude it. Under the peano axioms it’s explicitly spelled out:
0 is not the successor of any natural number
TeddE@lemmy.world 3 weeks ago
Ah! I see. Thanks for clarifying.
As to
m=s(n)andn=s(m), I think that is the motivation behind modular arithmetic and it gets used a lot with rotation, because 12 does loop back around to 1 in clocks, and a half turn to face backwards is the same position whether clockwise or counter. This is why we don’t use natural numbers for angles and use degrees and radians.I’m terms of parallels, I personally see that as a strength - instead of having successors (a term that intuitively embeds a concept of time/progression), I typically take the successor function as closer to the layman concept of ‘another’. Thus five bananas is
s(s(s(s(🍌))))and it does have a parallel to five carss(s(s(s(🚗)))). The fiveness doesn’t answer questions about the nature of the thing being counted (such as, "Are these cars: 🚓🚙🏎️🛵? "). Mathematicians like to use the size of the empty set as an abstract stand-in for when they don’t know what they’re talking about (in a literal sense, not broadly).As far as predecessors to 0 - undefined isn’t a problem for natural numbers, just for the people using them. And it makes a certain sense, too. You can’t actually have negative apples (regardless of how useful it may be to discuss a debt of apples).
anton@lemmy.blahaj.zone 3 weeks ago
But I am not taking about an amount of different things, but a parallel or branching number line being part of the set of natural numbers.
I am not talking about modular arithmetic on its own, but as part of the set of natural numbers.
Under the missing axioms those constructs would be part of the natural numbers, including an
xin N such thats(x)=xand thereforex+1=x. While some might think this implies0=1, it doesn’t, because we don’t have the axiom of induction, an thus can’t provea+c=b+c => a=b.The usefulness of such a system questionable but it certainly doesn’t describe the natural numbers as we understand them.
TeddE@lemmy.world 3 weeks ago
I apologize. I went back and reread from the top and I see my error.
My mobile Lemmy client indicates replies with cycling colors, and I had the misunderstanding that your objection was to the axioms presented in Principia Mathematica. But your reply was fair in the context of the axioms you were actually replying to.
anton@lemmy.blahaj.zone 3 weeks ago
While it was probably not the best use of our time, it certainly made me think about relations and algebra in more interesting ways than the last uni course did.