A maths joke I don't get

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jamespellis
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A maths joke I don't get

Postby jamespellis » Tue May 05, 2015 7:19 pm UTC

Does anyone "get" this joke?
"Let f(x) = c and g(x) = e^x. The product function fg is afraid of which differential operator?"
It was a joke made by my lecturer in some online notes and nobody I know can figure it out

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Gwydion
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Re: A maths joke I don't get

Postby Gwydion » Tue May 05, 2015 9:33 pm UTC

It's a really lame joke, but I think the answer is probably
Spoiler:
d/dy, as ce^x is not a varying function of y and therefore the partial derivative is 0. The "joke" coming from the fact that d(e^x)/dx is e^x, so e^x is "invincible" to differential operators.

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drachefly
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Re: A maths joke I don't get

Postby drachefly » Tue May 05, 2015 10:41 pm UTC

I'm not sure why they split it into two functions, then.

d/d(anything with no partial in respect to c and x) would leave them 0.

d/d(e^x) would result in c, a.k.a, f(x), and vice versa, but I don't see comedic potential.

f(x)g(x) = ce^x, which looks like it ought to be able to pun off of sex somehow, but not so much with 'fear'.

Are you sure this has been reproduced faithfully?

jamespellis
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Re: A maths joke I don't get

Postby jamespellis » Tue May 05, 2015 11:12 pm UTC

My best thought was a "second hand derivative" due to cex being a second hand type shop in the UK (where I live), but it doesn't explain being afraid.

I also considered sex jokes and came up short (I'm better at them clearly).

Partially differentiating by y may be the answer, but I'll be disappointed if it is.

It has been reproduced faithfully, I actually copied and pasted it from the 2013 exam for this course (I kid you not) where the answer was awarded 1 mark (no mark scheme given unfortunately).

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notzeb
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Re: A maths joke I don't get

Postby notzeb » Wed May 06, 2015 1:13 am UTC

(d/dx - 1) kills it, and doesn't rely on silly conventions like the d/dy answer (not everybody calls the output of a function "y", and for all we know x might be a function of y).
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jestingrabbit
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Re: A maths joke I don't get

Postby jestingrabbit » Wed May 06, 2015 3:23 am UTC

The fact that cex is a second hand store is interesting. You could differentiate wrt to c twice, which would be d2/dc2, and then it would be 0.

Or you could differentiate it once wrt c, and then it would be e^x.
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BedderDanu
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Re: A maths joke I don't get

Postby BedderDanu » Wed May 13, 2015 9:14 pm UTC

Assuming I'm remembering my DiffEQ class correctly:
_Dx_
. c-x
Spoiler:
Because it makes cex discontinuous.


...I'll see myself out.

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notzeb
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Re: A maths joke I don't get

Postby notzeb » Thu May 14, 2015 3:57 am UTC

There's also the trivial answer: 0. It's the differential operator that takes any function as input, and returns the (constant) 0 function as output.

Alternative answer: maybe by fg, your lecturer meant f∘g. Since (f∘g)(x) = c for all x, d/dx kills it. (So does 0, of course.)
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Re: A maths joke I don't get

Postby phlip » Thu May 14, 2015 7:08 am UTC

I'm not entirely sure what the joke in the OP is supposed to mean on its own, but it would make at least some amount of sense as a followup to this classic joke. Still not entirely sure what it would mean, but there would at least be some context to be had...

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