mersenneforum.org > Math Inverse of functions
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 2011-04-13, 17:31 #1 Raman Noodles     "Mr. Tuch" Dec 2007 Chennai, India 3·419 Posts Inverse of functions That website http://wims.unice.fr contains a plenty of useful online calculators, really a lot, but I couldn't find out a place where inverse of functions is being sought. For example take up with 1. $e^x + sinh(x)$ Can put with $sinh(x) = \frac{e^x - e^{-x}}{2}$ and then solve with that quadratic for $e^x$. The inverse is being given by $ln (\frac{x\pm\sqrt{x^2+3}}{3})$ 2. $cosh(x) + sinh(x)$ The inverse of this function is given by $ln\ x$ just simply this case goes away within that way. 3. $cos(x)+sin(x)$ Writing with this function as $\sqrt{2}\ sin(x+\frac{\pi}{4})$ The inverse can be given by $sin^{-1}\frac{x}{\sqrt 2}-\frac{\pi}{4}$ OK, then what will be the inverses of these following functions? It seems that they cannot be solved by using all those elementary mathematical functions at all! (4) $x^x$ (5) $x^{\frac{1}{x}}$ (6) $x+sin(x)$ (7) $x+tan(x)$ (8) $x+e^x$ (9) e$^x+sin(x)$ (10) $sin(x)+sinh(x)$ Thanks for your help, if possible within any case Disclaimer: I had faced a problem which asked me to give an algorithm to check out if N is a perfect power, xy where that value of y is ≥ 2. I told that it can be done by using checks of $\sqrt{x}$, $log_2(x)$, $^3\sqrt{x}$, $log_3(x)$, $^4\sqrt{x}$, $log_4(x)$, ... alternately, what value to check upto for base? Till N = zz, for some value of z. Thus, how to write up with that value of z as a function of N? How does that way work out rather... Last fiddled with by Raman on 2011-04-13 at 17:56
2011-04-13, 19:17   #2
CRGreathouse

Aug 2006

2·2,969 Posts

Quote:
 Originally Posted by Raman That website http://wims.unice.fr contains a plenty of useful online calculators, really a lot, but I couldn't find out a place where inverse of functions is being sought.
The problem is hard and in general cannot be done symbolically.

Quote:
 Originally Posted by Raman I had faced a problem which asked me to give an algorithm to check out if N is a perfect power, xy where that value of y is ≥ 2. I told that it can be done by using checks of $\sqrt{x}$, $log_2(x)$, $^3\sqrt{x}$, $log_3(x)$, $^4\sqrt{x}$, $log_4(x)$, ... alternately, what value to check upto for base? Till N = zz, for some value of z. Thus, how to write up with that value of z as a function of N? How does that way work out rather...
For that one you'll need a special function like Lambert's W, or numerical techniques. I think it's exp(W(log N)). W(x) is about log x - log log x, so this is roughly exp(log log N - log log log N) = log N / log log N, where log is the natural log.

Last fiddled with by CRGreathouse on 2011-04-13 at 19:20

2011-04-13, 19:58   #3
science_man_88

"Forget I exist"
Jul 2009
Dumbassville

8,369 Posts

Quote:
 Originally Posted by CRGreathouse The problem is hard and in general cannot be done symbolically. For that one you'll need a special function like Lambert's W, or numerical techniques. I think it's exp(W(log N)). W(x) is about log x - log log x, so this is roughly exp(log log N - log log log N) = log N / log log N, where log is the natural log.
if he didn't want work I'd say use PARI's ispower() with a loop through base values.

Last fiddled with by science_man_88 on 2011-04-13 at 19:58

2011-04-13, 23:05   #4
CRGreathouse

Aug 2006

2·2,969 Posts

Quote:
 Originally Posted by science_man_88 if he didn't want work I'd say use PARI's ispower() with a loop through base values.
With ispower() there's no need to loop at all -- ispower(N) does the right thing. ispower(N, b) is only when you want to check for a particular base and no others.

You should look at the code for it some time, it's fascinating. They check for 2nd, 3rd, and 5th powers separately, then move on to general testing for larger bases. Some amount of trial division is also done so you don't have to check quite as high, IIRC.

Incidentally, Bernstein has a nice paper about doing this test efficiently, in almost-linear time. Pretty impressive stuff.

Last fiddled with by CRGreathouse on 2011-04-13 at 23:06

2011-04-13, 23:22   #5
science_man_88

"Forget I exist"
Jul 2009
Dumbassville

836910 Posts

Quote:
 Originally Posted by CRGreathouse With ispower() there's no need to loop at all -- ispower(N) does the right thing. ispower(N, b) is only when you want to check for a particular base and no others. You should look at the code for it some time, it's fascinating. They check for 2nd, 3rd, and 5th powers separately, then move on to general testing for larger bases. Some amount of trial division is also done so you don't have to check quite as high, IIRC. Incidentally, Bernstein has a nice paper about doing this test efficiently, in almost-linear time. Pretty impressive stuff.
Bernstein ? I don't have a math degree and have never read the name to my knowledge.

2011-04-13, 23:29   #6
CRGreathouse

Aug 2006

2×2,969 Posts

Quote:
 Originally Posted by science_man_88 Bernstein ? I don't have a math degree and have never read the name to my knowledge.
http://cr.yp.to/

In particular, his math stuff is mostly in one of these:
http://cr.yp.to/ntheory.html
http://cr.yp.to/factorization.html
http://cr.yp.to/arith.html

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