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After seeing the following equation in a lecture about tensor analysis, I became confused.

$$ \frac{d\phi}{ds}=\frac{\partial \phi}{\partial x^m}\frac{dx^m}{ds} $$

What exactly is the difference between $d$ and $\partial$?

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Total derivative vs partial derivative. –  user2468 Aug 16 '12 at 18:28
    
@Jen, maybe elaborate a bit more in an answer? :) –  J. M. Aug 16 '12 at 18:33
    
I know, one is the partial and the other one is a total derivative. But isn't $\frac{\partial f}{\partial x}$ the same as $\frac{df}{dx}$? –  iblue Aug 16 '12 at 18:35
    
@iblue, the first one treats all the other independent variables as if they were constants. The second one doesn't. –  J. M. Aug 16 '12 at 18:40
    
@iblue Read what Jennifer linked to, it will help. –  Pedro Tamaroff Aug 16 '12 at 18:40
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2 Answers

up vote 2 down vote accepted

As mentioned $d$ means total and $\partial$ partial derivative and are not the same. Total derivative also counts $x$ dependencies in other variables. For instance: $$ f(x,v) = x^2 + v(x) \\ \frac{\partial f}{\partial x} = 2x \\ \frac{\partial f}{\partial v} = 1 \\ \frac{d f}{d x} = 2x + \frac{\partial v(x)}{\partial x} $$

Your formula most probably uses Einstein notation and is only a shorter way to write $$ \frac{d\phi}{ds}= \sum_m \frac{\partial \phi}{\partial x^m}\frac{dx^m}{ds} $$

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The difference is whether the rest of the variables of $f$ are considered constants or variables in $x.$ Former is partial, latter is total.

  • $\frac{\partial f}{\partial x}$ is the partial derivative: $f$ is differentiated w.r.t. to $x$ while all other variables are considered constants in $x.$

  • $\frac{d f}{d x}$ the is total derivative: $f$ is differentiated w.r.t. to $x$ while nothing is assumed about the other variables; they are considered variables in $x.$ (some variables might be, in fact, constants in $x.$)

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