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I have always written equations in the form $y=ax^2+bx+c$ but after entering an equation into Wolfram Alpha I noticed that the answer was displayed in the form $y=c+bx+ax^2$. I know that there is no mathematical difference between these two forms but was wondering why I learnt to write the higher powers first. I can't think of any immediate advantages of writing it either way.

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There's no difference and no advantage of one way over the other. –  egreg Nov 22 '13 at 11:44
    
-------why not? –  amatr Nov 22 '13 at 11:48
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As egreg said, no difference. But when you look at Taylor expansions, we use to start with the lowest degree because the terms use to become smaller and smaller. –  Claude Leibovici Nov 22 '13 at 11:48
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For the same reason business class board before coach. The most significant object comes first. In a Taylor series this is also true, the smaller the power the more important it gets. –  Asaf Karagila Nov 22 '13 at 11:54
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In my first algebra course, my teacher used to write a polynomial as $a_0 + a_1 x + \ldots$. The reason was that $\sum_{k=0}^n a_k x^k = a_0 x^0 + a_1 x^1 + \ldots + a_n x^n$. –  Siminore Nov 22 '13 at 11:58

2 Answers 2

up vote 2 down vote accepted

There's no difference and no general advantage of one way over the other. In some cases one direction may be preferable to the other, but just on psychological ground.

Writing polynomials starting from the higher or the lower degree term can give more prominence to one or the other term. For instance, when explaining division, it's better to start from the highest degree terms, because those are used for the first step. When writing down the division, who uses the Latin script sorts them from the highest degree term too, mainly because of the writing direction. The algorithm proceeds by lowering the degree, but it would be exactly the same if the polynomial were written in the opposite direction.

For series like $$ 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\dotsb $$ it's psychologically better writing the terms starting from the “lowest degree”, because the first terms show the pattern. In case of power series it would be no different writing $$ \dots+(-1)^n\frac{x^{2n+1}}{2n+1}+\dots+\frac{x^5}{5}-\frac{x^3}{3}+x $$ or $$ x-\frac{x^3}{3}+\frac{x^5}{5}-\frac{x^7}{7}+\dots+(-1)^n\frac{x^{2n+1}}{2n+1}+\dotsb $$ (there's a reason why I like this series ;-)), but it's quite clear that the latter form is better suited when using left to right writing.

When we write numbers using the Latin script, we start from the most significant digit and this might have influenced the order for polynomials. On the contrary, in Arabic script numbers are written starting from the least significant digit. Thus there is no necessity to prefer “small endian” to "big endian" or conversely.

Of course, the same considerations made for polynomials don't apply to numbers: in numbers we don't have the variable that makes clear what ordering we're using, so always sticking to one and the same ordering is a necessity.

When doing sums or multiplications between numbers, we follow a pattern going from the least significant digit to the most significant one, which is opposite to the direction of writing in Latin script, but in the same direction for the Arabic script. This is just a historic accident: when Fibonacci imported the numbering system he learnt from the Arabs, he didn't take into account the change in the direction of writing, so he continued to write the numbers in the same visual order as the Arabs'. In turn the Arabs didn't change the ordering when they imported the numbering system from the Indians (who write from left to right).

Why didn't they? One factor can be the way numbers are named: indoeuropean languages name the numbers mainly starting from the most significant digit (some exceptions are found, for instance drei und zwanzig in German), Arab instead starts from the least significant digit.

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Presumably for the same reason we write one-hundred as $100$ as opposed to $001$. The emphasis ought to be on the most significant digit, and in a polynomial the highest term is the "most powerful" and dominates all the other terms as we go to infinity.

Edit. Actually, it depends what we're trying to do. If we're trying to approximate a function near $x_0$ with a polynomial in the terms $(x-x_0)^n,$ then lower terms are more important. So perhaps in this case we should write

$$a+b(x-x_0)+c(x-x_0)^2$$

for better emphasis.

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Why is it the opposite convention for a power series? Where do you place the importance? –  Asaf Karagila Nov 22 '13 at 11:57
    
Arabs use the opposite convention: the most significant digit comes last in the direction of writing. –  egreg Nov 22 '13 at 11:59
    
@egreg: Arabic is also written in the opposite direction. (Which I am sure that you know, after reading your texts about using babel correctly.) –  Asaf Karagila Nov 22 '13 at 12:00
    
@AsafKaragila, I do see your point. If you're trying to approximate a function near $x_0$, then terms of the form $(x-x_0)^n$ are more significant for low $n$ than high $n$. I guess it depends what you're trying to do with it. –  goblin Nov 22 '13 at 12:01
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@tohecz: $\ldots+cx^2+bx+a$. Also fine. If you are going to insist that a series is of the form $\sum_{n=0}^\infty a_nx^n$, then to that I say - this is not a left to right, nor right to left notation. –  Asaf Karagila Nov 22 '13 at 12:14

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