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I've been interested in non-standard analysis recently. I was reading up on it and noticed the following interesting comment on the Wikipedia page about hyperreal numbers, right after giving an example of a nonstandard differentiation:

The use of the standard part in the definition of the derivative is a rigorous alternative to the traditional practice of neglecting the square of an infinitesimal quantity... the typical method from Newton through the 19th century would have been simply to discard the $dx^2$ term.

I've never heard anything like this before, and really find it fascinating that Newton's method was to define the relation $dx^2 = 0$. If we actually formalize the above structure by taking $\mathbb{R}$ and adjoining an element $dx^2 = 0$ to it, we get the "dual numbers," isomorphic to the quotient ring $\mathbb{R}[x]/x^2$. I'd seen some things about how this algebra plays into automated differentiation algorithms for some computer software systems, but I've never heard anything about Newton directly working in this algebra. So I have a few questions:

  1. Does anyone have more historical information on the way that Newton performed differentiation, and its relation to the dual numbers?
  2. Does anyone know how effectively real analysis can be formalized with the dual numbers? Does the resulting system play nice enough to develop all of the important modern results?
  3. If we start with $\mathbb{C}[x]/x^2$ instead, can we likewise develop complex analysis?

Since this idea is so simple, I'm very curious how powerful it is. I'm also curious if it has any major drawbacks too, since I'm not sure why anyone would mess with the foundational baggage involved in defining the hyperreals if this simple 2-dimensional real algebra could really do the trick.

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4 Answers 4

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The biggest draw back (and it's a big one) is that the ring of dual numbers is not a field. It has plenty of zero divisors. So, Newton, or any of the mathematicians of the early days of calculus, certainly did not work directly in the ring of dual numbers. They of course did not consider the ring to exist (as rings did not exist at all yet), but from their writing it is clear they envisaged a field of real numbers with, somehow, some notions of infinitesimals. Their work is of course very vague, but correct. Much more on that can be found in math history books. Many interesting discussions can be found in the recent book "Adventures in Formalism", also related to the early days of calculus and how things developed.

Some (rather unsatisfactory) portions of analysis can be developed in the ring of dual numbers, but it does not go too far. The idea, as you say, is very simple, perhaps too simple. One immediately gets into trouble when trying to define the derivative as the quotient of the infinitesimal $f(x+h)-f(x)$ divided by $h$, where $h$ is infinitesimal. The difficulty is that the non-zero infinitesimals in the ring of dual numbers are not invertible. So, it's the end of the party. (As you say though, some aspects of the party remain with automatic differentiation). In some sense, the dual numbers form a first order approximation to actual infinitesimals: The square of an infinitesimal is of an order of magnitude smaller than the infinitesimal you started with, but in the ring of dual numbers, the square of an 'infinitesimal' is precisely $0$. So, in a nonstandard model of the reals you have whole layers of infinitesimals. In the dual numbers there is only one layer, nothing in it is invertible, and they all square to $0$.

The book Models for smooth infinitesimal analysis explores many different models for analysis with infinitesimals. None of them is particularly simple.

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On the other hand, for a polynomial $f$ we have that $f(x+\varepsilon)-f(x)$ equals $f'(x) \cdot \varepsilon$, so that $f'(x)=\frac{f(x+\varepsilon)-f(x)}{\varepsilon}$ in $k[\varepsilon]/\varepsilon^2 [x]$. This also illustrates that the ring of dual numbers is useful in algebraic geometry. –  Martin Brandenburg Mar 26 '13 at 9:13
    
Thanks for the detailed response and the reference, which I'll definitely check out. A question: is there any utility in adjoining an element $\omega = \frac{1}{\epsilon}$ to the ring, which then has the property that $\omega^2 = \infty$ (in the extended real line sense)? This still wouldn't give you a field, but it would at least make the various dual numbers invertible, and having derivatives take values in $\mathbb{R} \cup {\infty}$ doesn't seem that far out there. –  Mike Battaglia Mar 26 '13 at 9:19
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Quick afterthought since I can't edit: That should say $\mathbb{R} \cup \{\infty, -\infty\}$ above. So you'd get numbers of the form $a + b\epsilon + c\omega$, where $\epsilon^2 = 0$ and $\omega^2 = \infty$ and $-\omega^2 = -\infty$. Not a field, but still seems possibly useful... –  Mike Battaglia Mar 26 '13 at 9:25
    
@MikeBattaglia, you will still have the problem of there not being any layers of infinitesimals. The dual number is a rather crude model for adjoining infinitesimals. As Martin explains in his answer the comment, the dual numbers do find applications in algebraic geometry. But for analysis, it does not look promising. –  Ittay Weiss Mar 26 '13 at 9:32
    
Ittay: I understand that the dual numbers are "crude" in that sense, and that the hyperreals have layers and layers of infinitesimals whereas the dual numbers just have a single $\epsilon$\ that squares to zero. But, are you saying that the lack of "layers" in the dual case is the problem that prevents it from doing analysis? I see why the zero divisors would cause a problem, but why the lack of layers specifically? –  Mike Battaglia Mar 27 '13 at 11:34

No for 1. and 3., this ring is not really useful in analysis. But it is quite important for analytical considerations in algebraic geometry, the main reason being that the scheme $\mathrm{Spec}(k[\varepsilon]/\varepsilon^2)$ classifies tangent vectors. This makes it possible to define the tangent space of arbitrary functors $F : \mathsf{CRing} \to \mathsf{Set}$ at some $x \in F(k)$, namely as the fiber of $F(k[\varepsilon]/\varepsilon^2) \to F(k)$ at $x$. There is no manifold which represents tangent vectors for manifolds, so this is the main difference.

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This seems like a great response but unfortunately I'm not able to understand it yet! I've just started Hartshorne now but I'm still working on sheaves and haven't gotten to schemes yet. I'm going to have to come back to this in a few weeks and see if it all makes sense then.. –  Mike Battaglia Mar 26 '13 at 9:23
    
Better look up the notion of a derivation and try to prove that homomorphisms of rings $A \to k[\varepsilon]/\varepsilon^2$ correspond 1:1 to pairs consisting of a homomorphism of rings $A \to k$ and a $k$-derivation $A \to k$. This is the observation on which everything else rests. –  Martin Brandenburg Mar 26 '13 at 9:51
    
@MikeBattaglia There is a related explanation of first order infinitesmial symmetries in one of Qiaochu Yuan's old blogposts that may or may not help you develop a picture of the above idea. –  rschwieb Mar 26 '13 at 10:01
    
Thanks for the references, everyone - will check those out. –  Mike Battaglia Mar 27 '13 at 21:20

The answers by Ittay Weiss and Martin Brandenburg are helpful. I would like to point out a more direct shorcoming of the dual numbers as far as analysis (and even calculus) is concerned is that it is not clear how to extend a generic real function to the dual numbers, even say a $C^\infty$ smooth function. Thus, if one wishes to form a ratio of infinitesimals involved in the definition of the derivative, it is not clear what should appear in the numerator. Over the hyperreals, one has a systematic way of extending every real function to the wider hyperreal domain, and the transfer principle (which is arguably a formalisation of the Leibnizian Law of Continuity) ensures that such an extension is meaningful.

For this reason, the answer to the original question would be: No, dual numbers are insufficient to capture "Newton's historical method for doing calculus". The hyperreals provide a framework where the procedures of 17th century infinitesimal calculus can be successfully formalized.

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I never saw this before, but since I'm seeing it now: I mentioned above, in my comments to Ittay, an algebraic structure where $\omega = 1/\epsilon$ is defined, with the property $\omega^2 = \infty$ and $-\omega^2 = -\infty$, taken from the extended real line $\mathbb{R} \cup \{\infty, -\infty\}$. Thus, every number in this system is of the form $a+b\epsilon+c\omega$, other than the special numbers $\infty$ and $-\infty$. This makes all sorts of things invertible which weren't invertible before. Would this still pose a problem in extending generic real functions to the dual numbers? –  Mike Battaglia Aug 16 '13 at 7:01
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@Mike: One immediate problem is that the condition $\epsilon^2=0$ would not allow us to carry out the usual algebraic simplifications on the relation $(\epsilon\omega)^2=1$. Thus one of the ordinary rules of algebra has to break down, which is inconvenient in a number system. In the hyperreal framework all ordinary rules of algebra continue to hold over the extended domain. –  user72694 Aug 16 '13 at 7:49
    
Aha, great point. I never saw that before. Thanks. –  Mike Battaglia Aug 16 '13 at 8:02

I think no one mentioned Synthetic differential geometry, there you have nonzero quantities with $dx^2=0$. For a very readable introduction I suggest:

Bell, A primer of infinitesimal Analysis

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Note that these quantities aren't actually nonzero. What one can say about them is that one cannot prove that they are zero; nor can one prove that they are nonzero. This is of course only possible when the background logic is intuitionistic. –  user72694 Nov 28 '13 at 19:14
    
Right. But at least one can prove that the set of all nilsquare elements does not reduce to the set $\{0\}$ –  Michael Nov 28 '13 at 21:44
    
How does that go? Is this really a "set" in the traditional sense? I recall that one needs a more sophisticated topos theory setting to make this work. Does one get a nonempty set of which one cannot exhibit any element? –  user72694 Nov 29 '13 at 8:22
    
@user72694 have a look at page 5 of these notes: home.sandiego.edu/~shulman/papers/sdg-pizza-seminar.pdf –  Michael Nov 29 '13 at 17:39

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