Getting stuck on simple logarithmic equation: $x times ln (x) = 1$












1












$begingroup$


$$x times ln (x) = 1$$



I am trying to solve that equation. I used the theory $ln(a) = ln(b)$ being equivalent to $a = b$ and got stuck at



$$x = e^{frac{1}{x}}$$



That's as far as I went and I know there's a solution (around 1.8 or 1.9), since I used my calculator, but I'd like to know how to do this by hand.










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$endgroup$












  • $begingroup$
    You'll probably have to use the Lambert W function.
    $endgroup$
    – Alex Becker
    Apr 8 '13 at 21:32










  • $begingroup$
    This is not an equation that can be solved with elementary functions, as far as I can tell.
    $endgroup$
    – Sammy Black
    Apr 8 '13 at 21:34
















1












$begingroup$


$$x times ln (x) = 1$$



I am trying to solve that equation. I used the theory $ln(a) = ln(b)$ being equivalent to $a = b$ and got stuck at



$$x = e^{frac{1}{x}}$$



That's as far as I went and I know there's a solution (around 1.8 or 1.9), since I used my calculator, but I'd like to know how to do this by hand.










share|cite|improve this question











$endgroup$












  • $begingroup$
    You'll probably have to use the Lambert W function.
    $endgroup$
    – Alex Becker
    Apr 8 '13 at 21:32










  • $begingroup$
    This is not an equation that can be solved with elementary functions, as far as I can tell.
    $endgroup$
    – Sammy Black
    Apr 8 '13 at 21:34














1












1








1





$begingroup$


$$x times ln (x) = 1$$



I am trying to solve that equation. I used the theory $ln(a) = ln(b)$ being equivalent to $a = b$ and got stuck at



$$x = e^{frac{1}{x}}$$



That's as far as I went and I know there's a solution (around 1.8 or 1.9), since I used my calculator, but I'd like to know how to do this by hand.










share|cite|improve this question











$endgroup$




$$x times ln (x) = 1$$



I am trying to solve that equation. I used the theory $ln(a) = ln(b)$ being equivalent to $a = b$ and got stuck at



$$x = e^{frac{1}{x}}$$



That's as far as I went and I know there's a solution (around 1.8 or 1.9), since I used my calculator, but I'd like to know how to do this by hand.







logarithms






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share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Dec 7 '18 at 12:26









Martin Sleziak

44.8k9118272




44.8k9118272










asked Apr 8 '13 at 21:28









David GomesDavid Gomes

122127




122127












  • $begingroup$
    You'll probably have to use the Lambert W function.
    $endgroup$
    – Alex Becker
    Apr 8 '13 at 21:32










  • $begingroup$
    This is not an equation that can be solved with elementary functions, as far as I can tell.
    $endgroup$
    – Sammy Black
    Apr 8 '13 at 21:34


















  • $begingroup$
    You'll probably have to use the Lambert W function.
    $endgroup$
    – Alex Becker
    Apr 8 '13 at 21:32










  • $begingroup$
    This is not an equation that can be solved with elementary functions, as far as I can tell.
    $endgroup$
    – Sammy Black
    Apr 8 '13 at 21:34
















$begingroup$
You'll probably have to use the Lambert W function.
$endgroup$
– Alex Becker
Apr 8 '13 at 21:32




$begingroup$
You'll probably have to use the Lambert W function.
$endgroup$
– Alex Becker
Apr 8 '13 at 21:32












$begingroup$
This is not an equation that can be solved with elementary functions, as far as I can tell.
$endgroup$
– Sammy Black
Apr 8 '13 at 21:34




$begingroup$
This is not an equation that can be solved with elementary functions, as far as I can tell.
$endgroup$
– Sammy Black
Apr 8 '13 at 21:34










5 Answers
5






active

oldest

votes


















6












$begingroup$

You can use the law of logarithms which states that for $a,binmathbb{R}$: $aln{b}=lnleft(b^{a}right)$.



Therefore, you have:



$$xln{x}=1 implies ln{x^{x}}=1$$



You hence have:



$$x^{x}=e$$



Which does not have an elementary closed form, so you must use numerical methods (for instance Newton-Raphson iteration) to get an approximation (Mathematica gives $xapprox 1.76322$).



If you're interested, the closed form solution is: $$frac{1}{W(1)}, qquad text{ where } W(z) text{ is the LambertW function}$$






share|cite|improve this answer









$endgroup$





















    1












    $begingroup$

    There is no solution using only algebraic manipulation. We have to use the "product-log" or Lambert-W function to solve this, and this function doesn't fall in the "simple functions" category. :)



    Basically, the Lambert-W function is the inverse function of:
    $$f(x) = xe^x$$
    Equivalently:
    $$x=W(xe^x)$$



    So, using your expression:
    $$x=e^frac{1}{x}$$
    $$1=frac{1}{x}e^frac{1}{x}$$
    Taking the product-log of both sides:
    $$W(1)=Wleft(frac{1}{x}e^frac{1}{x}right)$$
    $$W(1)=frac{1}{x}$$
    $$x=frac{1}{W(1)}$$






    share|cite|improve this answer









    $endgroup$





















      0












      $begingroup$

      This equation won't have an elementary solution - you'll have to solve it numerically. (Or you can ask WA).






      share|cite|improve this answer









      $endgroup$





















        0












        $begingroup$

        It's in fact impossible to express your variable $x$ as a combination of usual elementary function. Symbolically, you shall express it using Lambert's L function, which solves the equation



        $$x=L(x)e^{L(x)}$$



        For instance, if $$xe^x = 1$$ then $x=L(1)$ as is easily seen. By expressing $x$ as a the logarithm of some other number $y$, one has from the preceding equation



        $$yln y=1$$



        Then, the solution of your equation would be $ln(y)=L(1)$ or $y=e^{L(1)}$. Take a look at wikipedia's http://en.wikipedia.org/wiki/Lambert_W_function for further information!






        share|cite|improve this answer









        $endgroup$













        • $begingroup$
          $L{}$ function?
          $endgroup$
          – apnorton
          Apr 8 '13 at 21:52



















        -1












        $begingroup$

        ${}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}$See Example 4 here: http://en.wikipedia.org/wiki/Lambert_W_function






        share|cite|improve this answer









        $endgroup$









        • 1




          $begingroup$
          (Not my downvote.) The community typically likes answers that are self-contained, rather than just links to outside websites. This might work well as a comment, though.
          $endgroup$
          – apnorton
          Apr 8 '13 at 21:38












        • $begingroup$
          @anorton Thanks for letting me know. I think that the criterion does not make sense (even though I will respect that), especially if that outside website is Wikipedia, and the link contains the answer + other things that the OP may want to know about the problem he/she is dealing with. The "downvote and go," that is what I do not understand (not that I care about my "score" here).
          $endgroup$
          – Lord Soth
          Apr 8 '13 at 21:53










        • $begingroup$
          I understand. I also dislike it when someone just downvotes one of my answers without any feedback whatsoever. I think the reasoning for self-contained answers is that links can go bad over time (for example, someone could renumber the examples on Wikipedia), but answers here should be time-independent.
          $endgroup$
          – apnorton
          Apr 8 '13 at 21:55











        Your Answer





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        5 Answers
        5






        active

        oldest

        votes








        5 Answers
        5






        active

        oldest

        votes









        active

        oldest

        votes






        active

        oldest

        votes









        6












        $begingroup$

        You can use the law of logarithms which states that for $a,binmathbb{R}$: $aln{b}=lnleft(b^{a}right)$.



        Therefore, you have:



        $$xln{x}=1 implies ln{x^{x}}=1$$



        You hence have:



        $$x^{x}=e$$



        Which does not have an elementary closed form, so you must use numerical methods (for instance Newton-Raphson iteration) to get an approximation (Mathematica gives $xapprox 1.76322$).



        If you're interested, the closed form solution is: $$frac{1}{W(1)}, qquad text{ where } W(z) text{ is the LambertW function}$$






        share|cite|improve this answer









        $endgroup$


















          6












          $begingroup$

          You can use the law of logarithms which states that for $a,binmathbb{R}$: $aln{b}=lnleft(b^{a}right)$.



          Therefore, you have:



          $$xln{x}=1 implies ln{x^{x}}=1$$



          You hence have:



          $$x^{x}=e$$



          Which does not have an elementary closed form, so you must use numerical methods (for instance Newton-Raphson iteration) to get an approximation (Mathematica gives $xapprox 1.76322$).



          If you're interested, the closed form solution is: $$frac{1}{W(1)}, qquad text{ where } W(z) text{ is the LambertW function}$$






          share|cite|improve this answer









          $endgroup$
















            6












            6








            6





            $begingroup$

            You can use the law of logarithms which states that for $a,binmathbb{R}$: $aln{b}=lnleft(b^{a}right)$.



            Therefore, you have:



            $$xln{x}=1 implies ln{x^{x}}=1$$



            You hence have:



            $$x^{x}=e$$



            Which does not have an elementary closed form, so you must use numerical methods (for instance Newton-Raphson iteration) to get an approximation (Mathematica gives $xapprox 1.76322$).



            If you're interested, the closed form solution is: $$frac{1}{W(1)}, qquad text{ where } W(z) text{ is the LambertW function}$$






            share|cite|improve this answer









            $endgroup$



            You can use the law of logarithms which states that for $a,binmathbb{R}$: $aln{b}=lnleft(b^{a}right)$.



            Therefore, you have:



            $$xln{x}=1 implies ln{x^{x}}=1$$



            You hence have:



            $$x^{x}=e$$



            Which does not have an elementary closed form, so you must use numerical methods (for instance Newton-Raphson iteration) to get an approximation (Mathematica gives $xapprox 1.76322$).



            If you're interested, the closed form solution is: $$frac{1}{W(1)}, qquad text{ where } W(z) text{ is the LambertW function}$$







            share|cite|improve this answer












            share|cite|improve this answer



            share|cite|improve this answer










            answered Apr 8 '13 at 21:32









            Thomas RussellThomas Russell

            7,82632551




            7,82632551























                1












                $begingroup$

                There is no solution using only algebraic manipulation. We have to use the "product-log" or Lambert-W function to solve this, and this function doesn't fall in the "simple functions" category. :)



                Basically, the Lambert-W function is the inverse function of:
                $$f(x) = xe^x$$
                Equivalently:
                $$x=W(xe^x)$$



                So, using your expression:
                $$x=e^frac{1}{x}$$
                $$1=frac{1}{x}e^frac{1}{x}$$
                Taking the product-log of both sides:
                $$W(1)=Wleft(frac{1}{x}e^frac{1}{x}right)$$
                $$W(1)=frac{1}{x}$$
                $$x=frac{1}{W(1)}$$






                share|cite|improve this answer









                $endgroup$


















                  1












                  $begingroup$

                  There is no solution using only algebraic manipulation. We have to use the "product-log" or Lambert-W function to solve this, and this function doesn't fall in the "simple functions" category. :)



                  Basically, the Lambert-W function is the inverse function of:
                  $$f(x) = xe^x$$
                  Equivalently:
                  $$x=W(xe^x)$$



                  So, using your expression:
                  $$x=e^frac{1}{x}$$
                  $$1=frac{1}{x}e^frac{1}{x}$$
                  Taking the product-log of both sides:
                  $$W(1)=Wleft(frac{1}{x}e^frac{1}{x}right)$$
                  $$W(1)=frac{1}{x}$$
                  $$x=frac{1}{W(1)}$$






                  share|cite|improve this answer









                  $endgroup$
















                    1












                    1








                    1





                    $begingroup$

                    There is no solution using only algebraic manipulation. We have to use the "product-log" or Lambert-W function to solve this, and this function doesn't fall in the "simple functions" category. :)



                    Basically, the Lambert-W function is the inverse function of:
                    $$f(x) = xe^x$$
                    Equivalently:
                    $$x=W(xe^x)$$



                    So, using your expression:
                    $$x=e^frac{1}{x}$$
                    $$1=frac{1}{x}e^frac{1}{x}$$
                    Taking the product-log of both sides:
                    $$W(1)=Wleft(frac{1}{x}e^frac{1}{x}right)$$
                    $$W(1)=frac{1}{x}$$
                    $$x=frac{1}{W(1)}$$






                    share|cite|improve this answer









                    $endgroup$



                    There is no solution using only algebraic manipulation. We have to use the "product-log" or Lambert-W function to solve this, and this function doesn't fall in the "simple functions" category. :)



                    Basically, the Lambert-W function is the inverse function of:
                    $$f(x) = xe^x$$
                    Equivalently:
                    $$x=W(xe^x)$$



                    So, using your expression:
                    $$x=e^frac{1}{x}$$
                    $$1=frac{1}{x}e^frac{1}{x}$$
                    Taking the product-log of both sides:
                    $$W(1)=Wleft(frac{1}{x}e^frac{1}{x}right)$$
                    $$W(1)=frac{1}{x}$$
                    $$x=frac{1}{W(1)}$$







                    share|cite|improve this answer












                    share|cite|improve this answer



                    share|cite|improve this answer










                    answered Apr 8 '13 at 21:37









                    apnortonapnorton

                    15.2k33796




                    15.2k33796























                        0












                        $begingroup$

                        This equation won't have an elementary solution - you'll have to solve it numerically. (Or you can ask WA).






                        share|cite|improve this answer









                        $endgroup$


















                          0












                          $begingroup$

                          This equation won't have an elementary solution - you'll have to solve it numerically. (Or you can ask WA).






                          share|cite|improve this answer









                          $endgroup$
















                            0












                            0








                            0





                            $begingroup$

                            This equation won't have an elementary solution - you'll have to solve it numerically. (Or you can ask WA).






                            share|cite|improve this answer









                            $endgroup$



                            This equation won't have an elementary solution - you'll have to solve it numerically. (Or you can ask WA).







                            share|cite|improve this answer












                            share|cite|improve this answer



                            share|cite|improve this answer










                            answered Apr 8 '13 at 21:31









                            icurays1icurays1

                            13.4k13155




                            13.4k13155























                                0












                                $begingroup$

                                It's in fact impossible to express your variable $x$ as a combination of usual elementary function. Symbolically, you shall express it using Lambert's L function, which solves the equation



                                $$x=L(x)e^{L(x)}$$



                                For instance, if $$xe^x = 1$$ then $x=L(1)$ as is easily seen. By expressing $x$ as a the logarithm of some other number $y$, one has from the preceding equation



                                $$yln y=1$$



                                Then, the solution of your equation would be $ln(y)=L(1)$ or $y=e^{L(1)}$. Take a look at wikipedia's http://en.wikipedia.org/wiki/Lambert_W_function for further information!






                                share|cite|improve this answer









                                $endgroup$













                                • $begingroup$
                                  $L{}$ function?
                                  $endgroup$
                                  – apnorton
                                  Apr 8 '13 at 21:52
















                                0












                                $begingroup$

                                It's in fact impossible to express your variable $x$ as a combination of usual elementary function. Symbolically, you shall express it using Lambert's L function, which solves the equation



                                $$x=L(x)e^{L(x)}$$



                                For instance, if $$xe^x = 1$$ then $x=L(1)$ as is easily seen. By expressing $x$ as a the logarithm of some other number $y$, one has from the preceding equation



                                $$yln y=1$$



                                Then, the solution of your equation would be $ln(y)=L(1)$ or $y=e^{L(1)}$. Take a look at wikipedia's http://en.wikipedia.org/wiki/Lambert_W_function for further information!






                                share|cite|improve this answer









                                $endgroup$













                                • $begingroup$
                                  $L{}$ function?
                                  $endgroup$
                                  – apnorton
                                  Apr 8 '13 at 21:52














                                0












                                0








                                0





                                $begingroup$

                                It's in fact impossible to express your variable $x$ as a combination of usual elementary function. Symbolically, you shall express it using Lambert's L function, which solves the equation



                                $$x=L(x)e^{L(x)}$$



                                For instance, if $$xe^x = 1$$ then $x=L(1)$ as is easily seen. By expressing $x$ as a the logarithm of some other number $y$, one has from the preceding equation



                                $$yln y=1$$



                                Then, the solution of your equation would be $ln(y)=L(1)$ or $y=e^{L(1)}$. Take a look at wikipedia's http://en.wikipedia.org/wiki/Lambert_W_function for further information!






                                share|cite|improve this answer









                                $endgroup$



                                It's in fact impossible to express your variable $x$ as a combination of usual elementary function. Symbolically, you shall express it using Lambert's L function, which solves the equation



                                $$x=L(x)e^{L(x)}$$



                                For instance, if $$xe^x = 1$$ then $x=L(1)$ as is easily seen. By expressing $x$ as a the logarithm of some other number $y$, one has from the preceding equation



                                $$yln y=1$$



                                Then, the solution of your equation would be $ln(y)=L(1)$ or $y=e^{L(1)}$. Take a look at wikipedia's http://en.wikipedia.org/wiki/Lambert_W_function for further information!







                                share|cite|improve this answer












                                share|cite|improve this answer



                                share|cite|improve this answer










                                answered Apr 8 '13 at 21:50









                                paoloffpaoloff

                                262




                                262












                                • $begingroup$
                                  $L{}$ function?
                                  $endgroup$
                                  – apnorton
                                  Apr 8 '13 at 21:52


















                                • $begingroup$
                                  $L{}$ function?
                                  $endgroup$
                                  – apnorton
                                  Apr 8 '13 at 21:52
















                                $begingroup$
                                $L{}$ function?
                                $endgroup$
                                – apnorton
                                Apr 8 '13 at 21:52




                                $begingroup$
                                $L{}$ function?
                                $endgroup$
                                – apnorton
                                Apr 8 '13 at 21:52











                                -1












                                $begingroup$

                                ${}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}$See Example 4 here: http://en.wikipedia.org/wiki/Lambert_W_function






                                share|cite|improve this answer









                                $endgroup$









                                • 1




                                  $begingroup$
                                  (Not my downvote.) The community typically likes answers that are self-contained, rather than just links to outside websites. This might work well as a comment, though.
                                  $endgroup$
                                  – apnorton
                                  Apr 8 '13 at 21:38












                                • $begingroup$
                                  @anorton Thanks for letting me know. I think that the criterion does not make sense (even though I will respect that), especially if that outside website is Wikipedia, and the link contains the answer + other things that the OP may want to know about the problem he/she is dealing with. The "downvote and go," that is what I do not understand (not that I care about my "score" here).
                                  $endgroup$
                                  – Lord Soth
                                  Apr 8 '13 at 21:53










                                • $begingroup$
                                  I understand. I also dislike it when someone just downvotes one of my answers without any feedback whatsoever. I think the reasoning for self-contained answers is that links can go bad over time (for example, someone could renumber the examples on Wikipedia), but answers here should be time-independent.
                                  $endgroup$
                                  – apnorton
                                  Apr 8 '13 at 21:55
















                                -1












                                $begingroup$

                                ${}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}$See Example 4 here: http://en.wikipedia.org/wiki/Lambert_W_function






                                share|cite|improve this answer









                                $endgroup$









                                • 1




                                  $begingroup$
                                  (Not my downvote.) The community typically likes answers that are self-contained, rather than just links to outside websites. This might work well as a comment, though.
                                  $endgroup$
                                  – apnorton
                                  Apr 8 '13 at 21:38












                                • $begingroup$
                                  @anorton Thanks for letting me know. I think that the criterion does not make sense (even though I will respect that), especially if that outside website is Wikipedia, and the link contains the answer + other things that the OP may want to know about the problem he/she is dealing with. The "downvote and go," that is what I do not understand (not that I care about my "score" here).
                                  $endgroup$
                                  – Lord Soth
                                  Apr 8 '13 at 21:53










                                • $begingroup$
                                  I understand. I also dislike it when someone just downvotes one of my answers without any feedback whatsoever. I think the reasoning for self-contained answers is that links can go bad over time (for example, someone could renumber the examples on Wikipedia), but answers here should be time-independent.
                                  $endgroup$
                                  – apnorton
                                  Apr 8 '13 at 21:55














                                -1












                                -1








                                -1





                                $begingroup$

                                ${}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}$See Example 4 here: http://en.wikipedia.org/wiki/Lambert_W_function






                                share|cite|improve this answer









                                $endgroup$



                                ${}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}$See Example 4 here: http://en.wikipedia.org/wiki/Lambert_W_function







                                share|cite|improve this answer












                                share|cite|improve this answer



                                share|cite|improve this answer










                                answered Apr 8 '13 at 21:32









                                Lord SothLord Soth

                                6,5901235




                                6,5901235








                                • 1




                                  $begingroup$
                                  (Not my downvote.) The community typically likes answers that are self-contained, rather than just links to outside websites. This might work well as a comment, though.
                                  $endgroup$
                                  – apnorton
                                  Apr 8 '13 at 21:38












                                • $begingroup$
                                  @anorton Thanks for letting me know. I think that the criterion does not make sense (even though I will respect that), especially if that outside website is Wikipedia, and the link contains the answer + other things that the OP may want to know about the problem he/she is dealing with. The "downvote and go," that is what I do not understand (not that I care about my "score" here).
                                  $endgroup$
                                  – Lord Soth
                                  Apr 8 '13 at 21:53










                                • $begingroup$
                                  I understand. I also dislike it when someone just downvotes one of my answers without any feedback whatsoever. I think the reasoning for self-contained answers is that links can go bad over time (for example, someone could renumber the examples on Wikipedia), but answers here should be time-independent.
                                  $endgroup$
                                  – apnorton
                                  Apr 8 '13 at 21:55














                                • 1




                                  $begingroup$
                                  (Not my downvote.) The community typically likes answers that are self-contained, rather than just links to outside websites. This might work well as a comment, though.
                                  $endgroup$
                                  – apnorton
                                  Apr 8 '13 at 21:38












                                • $begingroup$
                                  @anorton Thanks for letting me know. I think that the criterion does not make sense (even though I will respect that), especially if that outside website is Wikipedia, and the link contains the answer + other things that the OP may want to know about the problem he/she is dealing with. The "downvote and go," that is what I do not understand (not that I care about my "score" here).
                                  $endgroup$
                                  – Lord Soth
                                  Apr 8 '13 at 21:53










                                • $begingroup$
                                  I understand. I also dislike it when someone just downvotes one of my answers without any feedback whatsoever. I think the reasoning for self-contained answers is that links can go bad over time (for example, someone could renumber the examples on Wikipedia), but answers here should be time-independent.
                                  $endgroup$
                                  – apnorton
                                  Apr 8 '13 at 21:55








                                1




                                1




                                $begingroup$
                                (Not my downvote.) The community typically likes answers that are self-contained, rather than just links to outside websites. This might work well as a comment, though.
                                $endgroup$
                                – apnorton
                                Apr 8 '13 at 21:38






                                $begingroup$
                                (Not my downvote.) The community typically likes answers that are self-contained, rather than just links to outside websites. This might work well as a comment, though.
                                $endgroup$
                                – apnorton
                                Apr 8 '13 at 21:38














                                $begingroup$
                                @anorton Thanks for letting me know. I think that the criterion does not make sense (even though I will respect that), especially if that outside website is Wikipedia, and the link contains the answer + other things that the OP may want to know about the problem he/she is dealing with. The "downvote and go," that is what I do not understand (not that I care about my "score" here).
                                $endgroup$
                                – Lord Soth
                                Apr 8 '13 at 21:53




                                $begingroup$
                                @anorton Thanks for letting me know. I think that the criterion does not make sense (even though I will respect that), especially if that outside website is Wikipedia, and the link contains the answer + other things that the OP may want to know about the problem he/she is dealing with. The "downvote and go," that is what I do not understand (not that I care about my "score" here).
                                $endgroup$
                                – Lord Soth
                                Apr 8 '13 at 21:53












                                $begingroup$
                                I understand. I also dislike it when someone just downvotes one of my answers without any feedback whatsoever. I think the reasoning for self-contained answers is that links can go bad over time (for example, someone could renumber the examples on Wikipedia), but answers here should be time-independent.
                                $endgroup$
                                – apnorton
                                Apr 8 '13 at 21:55




                                $begingroup$
                                I understand. I also dislike it when someone just downvotes one of my answers without any feedback whatsoever. I think the reasoning for self-contained answers is that links can go bad over time (for example, someone could renumber the examples on Wikipedia), but answers here should be time-independent.
                                $endgroup$
                                – apnorton
                                Apr 8 '13 at 21:55


















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