Motivation of the Logarithm












0












$begingroup$


Suppose that someone wants to calculate approximately the product of 101,123,958,959,055 and 342,234,234,234,236 without using a computer. Since these numbers are so long, completely carrying out multiplication by hand would take a long time because each digit in one number must be multiplied by every digit in the other number. I understand how a logarithm together with a logarithm table can help approximate this product, but why is this better than approximating it by the product of 101,000,000,000,000 and 342,000,000,000,000 (or some other number of the digits replaced by zeros)?










share|cite|improve this question











$endgroup$








  • 1




    $begingroup$
    See History of logarithms : "Mathematical tables containing common logarithms (base-10) were extensively used in computations prior to the advent of computers and calculators, not only because logarithms convert problems of multiplication and division into much easier addition and subtraction problems, but for an additional property ..."
    $endgroup$
    – Mauro ALLEGRANZA
    Jan 20 at 17:28












  • $begingroup$
    @MauroALLEGRANZA That Wikipedia page does address my question.
    $endgroup$
    – LinearGuy
    Jan 20 at 17:46










  • $begingroup$
    They were used mainly in astronomy for simplfying trigonometric calculations (for e.g. astronomical tables) : it is much more easy to sum a pair of number with 10 digits than multiply them...
    $endgroup$
    – Mauro ALLEGRANZA
    Jan 20 at 17:49










  • $begingroup$
    Noone at present can defend the idea that logarithms are a good way to compute products, especially with many digits... although as @Mauro ALLEGRANZA recalls it, it is the first reason for having recognized this function (which in fact has become a function 150 years after having been the object of intensive computations. I see at least 2 motivations for studying logarithm function : 1) to fill a gap in the list of antiderivatives of functions $x to x^{alpha}$ which are $frac{x^{alpha+1}}{alpha+1}$ except for $alpha=-1$ 2) as the reciprocal function of the fundamental exponential funct.
    $endgroup$
    – Jean Marie
    Jan 20 at 17:50












  • $begingroup$
    See also the post What sorts of calculations called for the invention of logarithm tables?
    $endgroup$
    – Mauro ALLEGRANZA
    Jan 20 at 17:51


















0












$begingroup$


Suppose that someone wants to calculate approximately the product of 101,123,958,959,055 and 342,234,234,234,236 without using a computer. Since these numbers are so long, completely carrying out multiplication by hand would take a long time because each digit in one number must be multiplied by every digit in the other number. I understand how a logarithm together with a logarithm table can help approximate this product, but why is this better than approximating it by the product of 101,000,000,000,000 and 342,000,000,000,000 (or some other number of the digits replaced by zeros)?










share|cite|improve this question











$endgroup$








  • 1




    $begingroup$
    See History of logarithms : "Mathematical tables containing common logarithms (base-10) were extensively used in computations prior to the advent of computers and calculators, not only because logarithms convert problems of multiplication and division into much easier addition and subtraction problems, but for an additional property ..."
    $endgroup$
    – Mauro ALLEGRANZA
    Jan 20 at 17:28












  • $begingroup$
    @MauroALLEGRANZA That Wikipedia page does address my question.
    $endgroup$
    – LinearGuy
    Jan 20 at 17:46










  • $begingroup$
    They were used mainly in astronomy for simplfying trigonometric calculations (for e.g. astronomical tables) : it is much more easy to sum a pair of number with 10 digits than multiply them...
    $endgroup$
    – Mauro ALLEGRANZA
    Jan 20 at 17:49










  • $begingroup$
    Noone at present can defend the idea that logarithms are a good way to compute products, especially with many digits... although as @Mauro ALLEGRANZA recalls it, it is the first reason for having recognized this function (which in fact has become a function 150 years after having been the object of intensive computations. I see at least 2 motivations for studying logarithm function : 1) to fill a gap in the list of antiderivatives of functions $x to x^{alpha}$ which are $frac{x^{alpha+1}}{alpha+1}$ except for $alpha=-1$ 2) as the reciprocal function of the fundamental exponential funct.
    $endgroup$
    – Jean Marie
    Jan 20 at 17:50












  • $begingroup$
    See also the post What sorts of calculations called for the invention of logarithm tables?
    $endgroup$
    – Mauro ALLEGRANZA
    Jan 20 at 17:51
















0












0








0


1



$begingroup$


Suppose that someone wants to calculate approximately the product of 101,123,958,959,055 and 342,234,234,234,236 without using a computer. Since these numbers are so long, completely carrying out multiplication by hand would take a long time because each digit in one number must be multiplied by every digit in the other number. I understand how a logarithm together with a logarithm table can help approximate this product, but why is this better than approximating it by the product of 101,000,000,000,000 and 342,000,000,000,000 (or some other number of the digits replaced by zeros)?










share|cite|improve this question











$endgroup$




Suppose that someone wants to calculate approximately the product of 101,123,958,959,055 and 342,234,234,234,236 without using a computer. Since these numbers are so long, completely carrying out multiplication by hand would take a long time because each digit in one number must be multiplied by every digit in the other number. I understand how a logarithm together with a logarithm table can help approximate this product, but why is this better than approximating it by the product of 101,000,000,000,000 and 342,000,000,000,000 (or some other number of the digits replaced by zeros)?







logarithms math-history motivation






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Jan 20 at 17:43







LinearGuy

















asked Jan 20 at 17:24









LinearGuyLinearGuy

1511




1511








  • 1




    $begingroup$
    See History of logarithms : "Mathematical tables containing common logarithms (base-10) were extensively used in computations prior to the advent of computers and calculators, not only because logarithms convert problems of multiplication and division into much easier addition and subtraction problems, but for an additional property ..."
    $endgroup$
    – Mauro ALLEGRANZA
    Jan 20 at 17:28












  • $begingroup$
    @MauroALLEGRANZA That Wikipedia page does address my question.
    $endgroup$
    – LinearGuy
    Jan 20 at 17:46










  • $begingroup$
    They were used mainly in astronomy for simplfying trigonometric calculations (for e.g. astronomical tables) : it is much more easy to sum a pair of number with 10 digits than multiply them...
    $endgroup$
    – Mauro ALLEGRANZA
    Jan 20 at 17:49










  • $begingroup$
    Noone at present can defend the idea that logarithms are a good way to compute products, especially with many digits... although as @Mauro ALLEGRANZA recalls it, it is the first reason for having recognized this function (which in fact has become a function 150 years after having been the object of intensive computations. I see at least 2 motivations for studying logarithm function : 1) to fill a gap in the list of antiderivatives of functions $x to x^{alpha}$ which are $frac{x^{alpha+1}}{alpha+1}$ except for $alpha=-1$ 2) as the reciprocal function of the fundamental exponential funct.
    $endgroup$
    – Jean Marie
    Jan 20 at 17:50












  • $begingroup$
    See also the post What sorts of calculations called for the invention of logarithm tables?
    $endgroup$
    – Mauro ALLEGRANZA
    Jan 20 at 17:51
















  • 1




    $begingroup$
    See History of logarithms : "Mathematical tables containing common logarithms (base-10) were extensively used in computations prior to the advent of computers and calculators, not only because logarithms convert problems of multiplication and division into much easier addition and subtraction problems, but for an additional property ..."
    $endgroup$
    – Mauro ALLEGRANZA
    Jan 20 at 17:28












  • $begingroup$
    @MauroALLEGRANZA That Wikipedia page does address my question.
    $endgroup$
    – LinearGuy
    Jan 20 at 17:46










  • $begingroup$
    They were used mainly in astronomy for simplfying trigonometric calculations (for e.g. astronomical tables) : it is much more easy to sum a pair of number with 10 digits than multiply them...
    $endgroup$
    – Mauro ALLEGRANZA
    Jan 20 at 17:49










  • $begingroup$
    Noone at present can defend the idea that logarithms are a good way to compute products, especially with many digits... although as @Mauro ALLEGRANZA recalls it, it is the first reason for having recognized this function (which in fact has become a function 150 years after having been the object of intensive computations. I see at least 2 motivations for studying logarithm function : 1) to fill a gap in the list of antiderivatives of functions $x to x^{alpha}$ which are $frac{x^{alpha+1}}{alpha+1}$ except for $alpha=-1$ 2) as the reciprocal function of the fundamental exponential funct.
    $endgroup$
    – Jean Marie
    Jan 20 at 17:50












  • $begingroup$
    See also the post What sorts of calculations called for the invention of logarithm tables?
    $endgroup$
    – Mauro ALLEGRANZA
    Jan 20 at 17:51










1




1




$begingroup$
See History of logarithms : "Mathematical tables containing common logarithms (base-10) were extensively used in computations prior to the advent of computers and calculators, not only because logarithms convert problems of multiplication and division into much easier addition and subtraction problems, but for an additional property ..."
$endgroup$
– Mauro ALLEGRANZA
Jan 20 at 17:28






$begingroup$
See History of logarithms : "Mathematical tables containing common logarithms (base-10) were extensively used in computations prior to the advent of computers and calculators, not only because logarithms convert problems of multiplication and division into much easier addition and subtraction problems, but for an additional property ..."
$endgroup$
– Mauro ALLEGRANZA
Jan 20 at 17:28














$begingroup$
@MauroALLEGRANZA That Wikipedia page does address my question.
$endgroup$
– LinearGuy
Jan 20 at 17:46




$begingroup$
@MauroALLEGRANZA That Wikipedia page does address my question.
$endgroup$
– LinearGuy
Jan 20 at 17:46












$begingroup$
They were used mainly in astronomy for simplfying trigonometric calculations (for e.g. astronomical tables) : it is much more easy to sum a pair of number with 10 digits than multiply them...
$endgroup$
– Mauro ALLEGRANZA
Jan 20 at 17:49




$begingroup$
They were used mainly in astronomy for simplfying trigonometric calculations (for e.g. astronomical tables) : it is much more easy to sum a pair of number with 10 digits than multiply them...
$endgroup$
– Mauro ALLEGRANZA
Jan 20 at 17:49












$begingroup$
Noone at present can defend the idea that logarithms are a good way to compute products, especially with many digits... although as @Mauro ALLEGRANZA recalls it, it is the first reason for having recognized this function (which in fact has become a function 150 years after having been the object of intensive computations. I see at least 2 motivations for studying logarithm function : 1) to fill a gap in the list of antiderivatives of functions $x to x^{alpha}$ which are $frac{x^{alpha+1}}{alpha+1}$ except for $alpha=-1$ 2) as the reciprocal function of the fundamental exponential funct.
$endgroup$
– Jean Marie
Jan 20 at 17:50






$begingroup$
Noone at present can defend the idea that logarithms are a good way to compute products, especially with many digits... although as @Mauro ALLEGRANZA recalls it, it is the first reason for having recognized this function (which in fact has become a function 150 years after having been the object of intensive computations. I see at least 2 motivations for studying logarithm function : 1) to fill a gap in the list of antiderivatives of functions $x to x^{alpha}$ which are $frac{x^{alpha+1}}{alpha+1}$ except for $alpha=-1$ 2) as the reciprocal function of the fundamental exponential funct.
$endgroup$
– Jean Marie
Jan 20 at 17:50














$begingroup$
See also the post What sorts of calculations called for the invention of logarithm tables?
$endgroup$
– Mauro ALLEGRANZA
Jan 20 at 17:51






$begingroup$
See also the post What sorts of calculations called for the invention of logarithm tables?
$endgroup$
– Mauro ALLEGRANZA
Jan 20 at 17:51












2 Answers
2






active

oldest

votes


















1












$begingroup$

Logarithms were not motivated by the desire to get exact solutions to problems such as multiplying two numbers with $15$ significant digits each.



With a decent table of logarithms you might have a precision of five digits, so indeed the answer you would get by using logarithms to compute
$101{,}123{,}958{,}959{,}055 times 342{,}234{,}234{,}234{,}236$
would be no more accurate than if you simply multiplied
$101{,}120{,}000{,}000{,}000 times 342{,}230{,}000{,}000{,}000.$
It might be possible to go to more digits with a better table and sophisticated interpolation techniques, but not many more digits.



Adding two five digit numbers is still easier to do than multiplying two five-digit numbers if the only tools at your disposal are pencil and paper.



This still might not have been enough motivation to purchase a table of logarithms if you only needed to do such a multiplication once a year. But there are problems that require you to perform many multiplications to get just one answer.
Therefore there was an incentive to develop and publish tables of logarithms and promote their use.



Meanwhile, logarithms also turned out to have some interesting mathematical properties, for example providing a solution for the integral of $frac1x$
(the only power of $x$ whose integral is not another power of $x$).
I think the main reason we study logarithms today is because of things like that, not because we want to get the product of two $n$-digit numbers.






share|cite|improve this answer









$endgroup$





















    -1












    $begingroup$

    Direct multiplication of two $n$ digits numbers takes on the order of $n^2$ elementary operations (digits multiplies and additions with carries).



    When they were performed with tables of logarithms, full $n$ digits were obtained, using high-precision tables plus interpolation techniques. Then additions were performed with just order $n$ elementary operations, and the total effort was more acceptable.






    share|cite|improve this answer









    $endgroup$













    • $begingroup$
      Will the downvoter care to explain ?
      $endgroup$
      – Yves Daoust
      Jan 20 at 17:59











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    2 Answers
    2






    active

    oldest

    votes








    2 Answers
    2






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    1












    $begingroup$

    Logarithms were not motivated by the desire to get exact solutions to problems such as multiplying two numbers with $15$ significant digits each.



    With a decent table of logarithms you might have a precision of five digits, so indeed the answer you would get by using logarithms to compute
    $101{,}123{,}958{,}959{,}055 times 342{,}234{,}234{,}234{,}236$
    would be no more accurate than if you simply multiplied
    $101{,}120{,}000{,}000{,}000 times 342{,}230{,}000{,}000{,}000.$
    It might be possible to go to more digits with a better table and sophisticated interpolation techniques, but not many more digits.



    Adding two five digit numbers is still easier to do than multiplying two five-digit numbers if the only tools at your disposal are pencil and paper.



    This still might not have been enough motivation to purchase a table of logarithms if you only needed to do such a multiplication once a year. But there are problems that require you to perform many multiplications to get just one answer.
    Therefore there was an incentive to develop and publish tables of logarithms and promote their use.



    Meanwhile, logarithms also turned out to have some interesting mathematical properties, for example providing a solution for the integral of $frac1x$
    (the only power of $x$ whose integral is not another power of $x$).
    I think the main reason we study logarithms today is because of things like that, not because we want to get the product of two $n$-digit numbers.






    share|cite|improve this answer









    $endgroup$


















      1












      $begingroup$

      Logarithms were not motivated by the desire to get exact solutions to problems such as multiplying two numbers with $15$ significant digits each.



      With a decent table of logarithms you might have a precision of five digits, so indeed the answer you would get by using logarithms to compute
      $101{,}123{,}958{,}959{,}055 times 342{,}234{,}234{,}234{,}236$
      would be no more accurate than if you simply multiplied
      $101{,}120{,}000{,}000{,}000 times 342{,}230{,}000{,}000{,}000.$
      It might be possible to go to more digits with a better table and sophisticated interpolation techniques, but not many more digits.



      Adding two five digit numbers is still easier to do than multiplying two five-digit numbers if the only tools at your disposal are pencil and paper.



      This still might not have been enough motivation to purchase a table of logarithms if you only needed to do such a multiplication once a year. But there are problems that require you to perform many multiplications to get just one answer.
      Therefore there was an incentive to develop and publish tables of logarithms and promote their use.



      Meanwhile, logarithms also turned out to have some interesting mathematical properties, for example providing a solution for the integral of $frac1x$
      (the only power of $x$ whose integral is not another power of $x$).
      I think the main reason we study logarithms today is because of things like that, not because we want to get the product of two $n$-digit numbers.






      share|cite|improve this answer









      $endgroup$
















        1












        1








        1





        $begingroup$

        Logarithms were not motivated by the desire to get exact solutions to problems such as multiplying two numbers with $15$ significant digits each.



        With a decent table of logarithms you might have a precision of five digits, so indeed the answer you would get by using logarithms to compute
        $101{,}123{,}958{,}959{,}055 times 342{,}234{,}234{,}234{,}236$
        would be no more accurate than if you simply multiplied
        $101{,}120{,}000{,}000{,}000 times 342{,}230{,}000{,}000{,}000.$
        It might be possible to go to more digits with a better table and sophisticated interpolation techniques, but not many more digits.



        Adding two five digit numbers is still easier to do than multiplying two five-digit numbers if the only tools at your disposal are pencil and paper.



        This still might not have been enough motivation to purchase a table of logarithms if you only needed to do such a multiplication once a year. But there are problems that require you to perform many multiplications to get just one answer.
        Therefore there was an incentive to develop and publish tables of logarithms and promote their use.



        Meanwhile, logarithms also turned out to have some interesting mathematical properties, for example providing a solution for the integral of $frac1x$
        (the only power of $x$ whose integral is not another power of $x$).
        I think the main reason we study logarithms today is because of things like that, not because we want to get the product of two $n$-digit numbers.






        share|cite|improve this answer









        $endgroup$



        Logarithms were not motivated by the desire to get exact solutions to problems such as multiplying two numbers with $15$ significant digits each.



        With a decent table of logarithms you might have a precision of five digits, so indeed the answer you would get by using logarithms to compute
        $101{,}123{,}958{,}959{,}055 times 342{,}234{,}234{,}234{,}236$
        would be no more accurate than if you simply multiplied
        $101{,}120{,}000{,}000{,}000 times 342{,}230{,}000{,}000{,}000.$
        It might be possible to go to more digits with a better table and sophisticated interpolation techniques, but not many more digits.



        Adding two five digit numbers is still easier to do than multiplying two five-digit numbers if the only tools at your disposal are pencil and paper.



        This still might not have been enough motivation to purchase a table of logarithms if you only needed to do such a multiplication once a year. But there are problems that require you to perform many multiplications to get just one answer.
        Therefore there was an incentive to develop and publish tables of logarithms and promote their use.



        Meanwhile, logarithms also turned out to have some interesting mathematical properties, for example providing a solution for the integral of $frac1x$
        (the only power of $x$ whose integral is not another power of $x$).
        I think the main reason we study logarithms today is because of things like that, not because we want to get the product of two $n$-digit numbers.







        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered Jan 20 at 19:13









        David KDavid K

        54.9k343120




        54.9k343120























            -1












            $begingroup$

            Direct multiplication of two $n$ digits numbers takes on the order of $n^2$ elementary operations (digits multiplies and additions with carries).



            When they were performed with tables of logarithms, full $n$ digits were obtained, using high-precision tables plus interpolation techniques. Then additions were performed with just order $n$ elementary operations, and the total effort was more acceptable.






            share|cite|improve this answer









            $endgroup$













            • $begingroup$
              Will the downvoter care to explain ?
              $endgroup$
              – Yves Daoust
              Jan 20 at 17:59
















            -1












            $begingroup$

            Direct multiplication of two $n$ digits numbers takes on the order of $n^2$ elementary operations (digits multiplies and additions with carries).



            When they were performed with tables of logarithms, full $n$ digits were obtained, using high-precision tables plus interpolation techniques. Then additions were performed with just order $n$ elementary operations, and the total effort was more acceptable.






            share|cite|improve this answer









            $endgroup$













            • $begingroup$
              Will the downvoter care to explain ?
              $endgroup$
              – Yves Daoust
              Jan 20 at 17:59














            -1












            -1








            -1





            $begingroup$

            Direct multiplication of two $n$ digits numbers takes on the order of $n^2$ elementary operations (digits multiplies and additions with carries).



            When they were performed with tables of logarithms, full $n$ digits were obtained, using high-precision tables plus interpolation techniques. Then additions were performed with just order $n$ elementary operations, and the total effort was more acceptable.






            share|cite|improve this answer









            $endgroup$



            Direct multiplication of two $n$ digits numbers takes on the order of $n^2$ elementary operations (digits multiplies and additions with carries).



            When they were performed with tables of logarithms, full $n$ digits were obtained, using high-precision tables plus interpolation techniques. Then additions were performed with just order $n$ elementary operations, and the total effort was more acceptable.







            share|cite|improve this answer












            share|cite|improve this answer



            share|cite|improve this answer










            answered Jan 20 at 17:52









            Yves DaoustYves Daoust

            129k675227




            129k675227












            • $begingroup$
              Will the downvoter care to explain ?
              $endgroup$
              – Yves Daoust
              Jan 20 at 17:59


















            • $begingroup$
              Will the downvoter care to explain ?
              $endgroup$
              – Yves Daoust
              Jan 20 at 17:59
















            $begingroup$
            Will the downvoter care to explain ?
            $endgroup$
            – Yves Daoust
            Jan 20 at 17:59




            $begingroup$
            Will the downvoter care to explain ?
            $endgroup$
            – Yves Daoust
            Jan 20 at 17:59


















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