The most interesting thing about it is that at one point people thought it was a paradox.
It took lots of very intelligent mathematicians to realize that infinities are weird. Even today, it takes quite some schooling to come to terms with some of the _easy_ examples. For example, I expect that easily over 95% of university graduates disagree with the statement "There are as many rational numbers as there are prime numbers." . I even fear that is true for the easier to believe "0.9999999... = 1"
Thought Experiments in Mathematics: Gabriel's Horn
31–40 of 43 posts
Re: Thought Experiments in Mathematics: Gabriel's Horn
#32Earlier quoted context omitted.
There probably is _some_ mathematical formalism out there where there are less prime numbers than natural numbers. For example, most mathematicians would agree that there are as many reals in [0, 1] as there are in the real line. But according to a different definition of size, namely the most famous measure from measure theory, the Lebesgue measure --- the length of [0, 1] is 1 while the real line is undefined (I th…
The real line is defined, as Lebesgue measure spits out the extended reals.
Re: Thought Experiments in Mathematics: Gabriel's Horn
#33Re: Thought Experiments in Mathematics: Gabriel's Horn
#34Is this similar to the Dirac Delta function, which bounds a finite area, with infinite length? Could one say they are different dimensional analogues of each other?
Re: Thought Experiments in Mathematics: Gabriel's Horn
#35And this idea of finite area described by y = f(x) [between this curve and the x-axis, say], with infinite perimeter, amounts to just the same thing some function f whose integral over an infinite range (e.g., from x = 1 to infinity) is finite. If you prefer to think discretely, this is essentially the same phenomenon as an infinite series with finite sum (splitting our curve into the block from x = 1 to 2, the block from x = 2 to 3, the block from x = 3 to 4, etc., the total area is the sum of the series of areas in these blocks, while the total perimeter is automatically infinite as at least length 1 on top and bottom is contributed in each of these blocks). So, consider, for example, y = 1/x^2, from x = 1 to infinity: infinite perimeter but finite area (as its antiderivative is -1/x + C, which only increases by a finite amount over this range; in the same way, differentiating any function with a finite asymptote yields examples, and every example comes in essentially this fashion).
In moving to 3d and making a surface of revolution out of our starting curve, we are now integrating πf^2 instead of f itself, so we need f^2 to yield a convergent integral rather than f itself, but otherwise everything is just the same. Thus, we can get away with f(x) = 1/x, as in Gabriel's Horn, but also, just as well, with f(x) = x^{-p} for any p > 1/2.
Not that these are the only such things; again, any convergent infinite integral or series yields examples. So, just as well, y = 1/2^x or such things would work.
Re: Thought Experiments in Mathematics: Gabriel's Horn
#36Earlier quoted context omitted.
There probably is _some_ mathematical formalism out there where there are less prime numbers than natural numbers. For example, most mathematicians would agree that there are as many reals in [0, 1] as there are in the real line. But according to a different definition of size, namely the most famous measure from measure theory, the Lebesgue measure --- the length of [0, 1] is 1 while the real line is undefined (I th…
"There probably is _some_ mathematical formalism out there where there are less prime numbers than natural numbers." You could call Z[n] ?for any composite n? that (in Z[4], the multiplication table only contains 0, 1, and 2, so 3 is prime there; Z[p] for prime p gives you p different numbers and zero primes) but I think those are the only ones. If you accept that infinity exists you get Hilbert's hotel, which gets y…
I dont know that it would be exactly analogous to normal Z[M+1], and might have useful properties for some kind of geometric or combinatorical modeling. (My hunch is any time you want to be capable of carrying a "crossed threshold" flag as well as a value, and have that cascade through calculation.)
It would also model systems where you can get to infinity in finite steps, but can't traverse back. Not sure if those are useful in the abstract, though.
Re: Thought Experiments in Mathematics: Gabriel's Horn
#37Earlier quoted context omitted.
"There probably is _some_ mathematical formalism out there where there are less prime numbers than natural numbers." You could call Z[n] ?for any composite n? that (in Z[4], the multiplication table only contains 0, 1, and 2, so 3 is prime there; Z[p] for prime p gives you p different numbers and zero primes) but I think those are the only ones. If you accept that infinity exists you get Hilbert's hotel, which gets y…
Can you not make a "tropical Zn" with elements 0 - (M-1), and a "big" element, representing anything M or larger, including infinite values? I dont know that it would be exactly analogous to normal Z[M+1], and might have useful properties for some kind of geometric or combinatorical modeling. (My hunch is any time you want to be capable of carrying a "crossed threshold" flag as well as a value, and have that cascade…
That's pragmatic, highly useful, but a nightmare for mathematicians. You lose invariants such as x+y-y=x (associativity and communicativity, in general), so you're no longer talking of a group (I don't know of research on 'almost groups')
Compiler writers may happily make matters worse by assuming those laws still hold, with the effect that the same computation may overflow or not on different CPUs, under different compilers, compiler settings, or even the same compiler in the same compilation run.
Re: Thought Experiments in Mathematics: Gabriel's Horn
#38Earlier quoted context omitted.
Can you not make a "tropical Zn" with elements 0 - (M-1), and a "big" element, representing anything M or larger, including infinite values? I dont know that it would be exactly analogous to normal Z[M+1], and might have useful properties for some kind of geometric or combinatorical modeling. (My hunch is any time you want to be capable of carrying a "crossed threshold" flag as well as a value, and have that cascade…
IEEE 754 ( https://en.wikipedia.org/wiki/IEEE_floating_point ) does that (for floating point, but the integer variant would be easily defined), with a few extensions. You need "small", too, for example, and "don't have the faintest idea" for when somebody subtracts "big" from "big". That's pragmatic, highly useful, but a nightmare for mathematicians. You lose invariants such as x+y-y=x (associativity and communicativ…
Re: Thought Experiments in Mathematics: Gabriel's Horn
#39Earlier quoted context omitted.
Can you not make a "tropical Zn" with elements 0 - (M-1), and a "big" element, representing anything M or larger, including infinite values? I dont know that it would be exactly analogous to normal Z[M+1], and might have useful properties for some kind of geometric or combinatorical modeling. (My hunch is any time you want to be capable of carrying a "crossed threshold" flag as well as a value, and have that cascade…
IEEE 754 ( https://en.wikipedia.org/wiki/IEEE_floating_point ) does that (for floating point, but the integer variant would be easily defined), with a few extensions. You need "small", too, for example, and "don't have the faintest idea" for when somebody subtracts "big" from "big". That's pragmatic, highly useful, but a nightmare for mathematicians. You lose invariants such as x+y-y=x (associativity and communicativ…
Well, you actually just lose general inverses, so there's no sensible '-' operation that can just be turned in to '+ (-x)', where -x is the inverse of x under +. It's similar to the case where you have x * y / y = x except in the case where y = 0 (because there's no 0^-1). (As someone else pointed out, this puts you in to a semi-ring rather than a ring, which is what the general tropical semi-ring is. [1])
You can still have elements for which x + y - y = x is true, and usually that's a well-defined sub-space of x,y combinations. Proofs then usually use a case analysis: either we're in the subspace where x + y - y = x holds, or else we can use a property of not being in that space to derive a different but still useful conclusion.
In some senses, it acts as a Maybe type, and can perform arithmetic without having to unpack that Maybe-ness.
Re: Thought Experiments in Mathematics: Gabriel's Horn
#40Earlier quoted context omitted.
IEEE 754 ( https://en.wikipedia.org/wiki/IEEE_floating_point ) does that (for floating point, but the integer variant would be easily defined), with a few extensions. You need "small", too, for example, and "don't have the faintest idea" for when somebody subtracts "big" from "big". That's pragmatic, highly useful, but a nightmare for mathematicians. You lose invariants such as x+y-y=x (associativity and communicativ…
"almost groups" are called semigroups. Somehow people have written multiple books on the subjects.