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# function determination

Posted on 2010-11-20
Medium Priority
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Given that a function is defined with the following properties:-

f(xy) =  f(x) + f(y)

it can be seen that this is the case for f(x) = log x

Is there any way that f(x) can be deduced analytically rather than intuitively ?
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Question by:alcindor
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LVL 37

Expert Comment

ID: 34181723
Yeah. Just work it out.
log(xy)=log(x) + log(y) and manipulate it until the left and right match, or can't
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LVL 84

Accepted Solution

ozo earned 2000 total points
ID: 34181772
f(xy) =  f(x) + f(y)

f(x*1) = f(x) +      f(1)
f(1)=0
f(1) = f(x * 1/x) = f(x) + f(1/x)
f(x) = -f(1/x)

f(x+epsilon)-f(x)
=
f(x+epsilon)+f(1/x)
=
f((x+epsilon)/x)
=
f(1+epsilon/x)

If we assume that f is analytic:
f'(1)=lim(epsilon->0:f(1+epsilon)/epsilon)

f'(x)=
lim(epsilon->0:(f(x+epsilon)-f(x))/epsilon)
=
lim(epsilon->0:f(1+epsilon/x)/epsilon)
=
lim(epsilon->0:( f(1)+(epsilon/x)*f'(1))/epsilon)
=
f'(1)/x

so f(x) = integral(1..x:f'(1)/x)
0

LVL 27

Expert Comment

ID: 34181796
You can determine a few things analytically:

f(x*1)  = f(x) + (f(1)      ==>  f(1) = 0

You can pick any value of x>1 as the base of the log function.
You don't have to pick 10 or e.
Once you have a base, you can calculate lots of values.

f(x) =1      ==>  the base

f(x*x)  =  f(x) + f(x) = 2   ==>   f(x^n) = n

f(sqrt(x)) =  1/2

Once you have lots of values, you can find any value by interpolation.
0

LVL 84

Expert Comment

ID: 34181840
> you can find any value by interpolation.
assuming that f is analytic
0

LVL 2

Author Closing Comment

ID: 34183276
0

LVL 5

Expert Comment

ID: 34183355
Definition of logarithm:

Therefore:

And CQFD:

0

LVL 5

Expert Comment

ID: 34183362
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