Give a big-O estimate of $(x+1)mathrm{log}(x^2+1) + 3x^2$












1












$begingroup$


I wanted to know if the following solution demonstrates that the function $f(x) = (x+1)mathrm{log}, (x^2+1) + 3x^2 in O(x^2)$, because my answer and the book's answer deviate slightly.



Clearly,



$$3x^2 in O(x^2) tag{1}$$
$$x+1 in O(x)tag{2}$$



The following inequality is where the book and I differ, but I understand how the author obtained his big-O estimate. I said



$$mathrm{log}(x^2+1) in O(mathrm{log}(x^2+1))tag{3}$$



Therefore, the product of $(2)$ and $(3)$ renders



$$(x+1)mathrm{log}(x^2+1) in O(x mathrm{log}(x^2+1)) tag{3}$$



Finally, $(1)$ and $(3)$ gives us this big-O estimate



$$(x+1)mathrm{log}(x^2+1) in O(mathrm{max}(x^2,xmathrm{log}(x^2+1)) = O(x^2) tag{4}$$



Any problems! Thanks!










share|cite|improve this question











$endgroup$












  • $begingroup$
    It's "obvious" to me (especially after looking at a couple of graphs on a graphing calculator) that $log(x^2 + 1) < x$ for all $x > 0$, but I don't know whether that's enough proof for your purposes. I'm also puzzled why you introduced the symbol $n$ in place of $1$.
    $endgroup$
    – David K
    Feb 22 '16 at 4:24












  • $begingroup$
    @DavidK Substituting $n$ for 1 was a mistake. And yes, it is clear that $mathrm{log}(x^2+1) in O(x)$, but I put $O(mathrm{log}(x^2+1)$ instead, because that came to me first.
    $endgroup$
    – Benedict Voltaire
    Feb 22 '16 at 4:37










  • $begingroup$
    Do you mean as $x to 0$ or $x to infty$?
    $endgroup$
    – marty cohen
    Feb 22 '16 at 4:59










  • $begingroup$
    I think you meant to write ${}+3x^2$ on the left of $(4)$, but aside from that it looks OK now. As noted in the answer by marty cohen, you could simplify the $O(log x)$ factor a bit, but in the end all you need is for that part to be $O(x)$. Let me guess, the book went directly to $O(x)$ for that part rather than waiting until line $(4)$, was that the difference in their approach?
    $endgroup$
    – David K
    Feb 22 '16 at 13:29










  • $begingroup$
    Actually, the book has $mathrm{log}(x^2+1) in O(mathrm{log}(x))$ in substitution for my $(3)$. Is it more natural to adopt a slower function as the author does when dealing with big-O estimates, because that's the only reason I can think of for the author not using $mathrm{log}(x^2+1) in O(mathrm{log}(x^2+1))$.
    $endgroup$
    – Benedict Voltaire
    Feb 22 '16 at 16:30


















1












$begingroup$


I wanted to know if the following solution demonstrates that the function $f(x) = (x+1)mathrm{log}, (x^2+1) + 3x^2 in O(x^2)$, because my answer and the book's answer deviate slightly.



Clearly,



$$3x^2 in O(x^2) tag{1}$$
$$x+1 in O(x)tag{2}$$



The following inequality is where the book and I differ, but I understand how the author obtained his big-O estimate. I said



$$mathrm{log}(x^2+1) in O(mathrm{log}(x^2+1))tag{3}$$



Therefore, the product of $(2)$ and $(3)$ renders



$$(x+1)mathrm{log}(x^2+1) in O(x mathrm{log}(x^2+1)) tag{3}$$



Finally, $(1)$ and $(3)$ gives us this big-O estimate



$$(x+1)mathrm{log}(x^2+1) in O(mathrm{max}(x^2,xmathrm{log}(x^2+1)) = O(x^2) tag{4}$$



Any problems! Thanks!










share|cite|improve this question











$endgroup$












  • $begingroup$
    It's "obvious" to me (especially after looking at a couple of graphs on a graphing calculator) that $log(x^2 + 1) < x$ for all $x > 0$, but I don't know whether that's enough proof for your purposes. I'm also puzzled why you introduced the symbol $n$ in place of $1$.
    $endgroup$
    – David K
    Feb 22 '16 at 4:24












  • $begingroup$
    @DavidK Substituting $n$ for 1 was a mistake. And yes, it is clear that $mathrm{log}(x^2+1) in O(x)$, but I put $O(mathrm{log}(x^2+1)$ instead, because that came to me first.
    $endgroup$
    – Benedict Voltaire
    Feb 22 '16 at 4:37










  • $begingroup$
    Do you mean as $x to 0$ or $x to infty$?
    $endgroup$
    – marty cohen
    Feb 22 '16 at 4:59










  • $begingroup$
    I think you meant to write ${}+3x^2$ on the left of $(4)$, but aside from that it looks OK now. As noted in the answer by marty cohen, you could simplify the $O(log x)$ factor a bit, but in the end all you need is for that part to be $O(x)$. Let me guess, the book went directly to $O(x)$ for that part rather than waiting until line $(4)$, was that the difference in their approach?
    $endgroup$
    – David K
    Feb 22 '16 at 13:29










  • $begingroup$
    Actually, the book has $mathrm{log}(x^2+1) in O(mathrm{log}(x))$ in substitution for my $(3)$. Is it more natural to adopt a slower function as the author does when dealing with big-O estimates, because that's the only reason I can think of for the author not using $mathrm{log}(x^2+1) in O(mathrm{log}(x^2+1))$.
    $endgroup$
    – Benedict Voltaire
    Feb 22 '16 at 16:30
















1












1








1





$begingroup$


I wanted to know if the following solution demonstrates that the function $f(x) = (x+1)mathrm{log}, (x^2+1) + 3x^2 in O(x^2)$, because my answer and the book's answer deviate slightly.



Clearly,



$$3x^2 in O(x^2) tag{1}$$
$$x+1 in O(x)tag{2}$$



The following inequality is where the book and I differ, but I understand how the author obtained his big-O estimate. I said



$$mathrm{log}(x^2+1) in O(mathrm{log}(x^2+1))tag{3}$$



Therefore, the product of $(2)$ and $(3)$ renders



$$(x+1)mathrm{log}(x^2+1) in O(x mathrm{log}(x^2+1)) tag{3}$$



Finally, $(1)$ and $(3)$ gives us this big-O estimate



$$(x+1)mathrm{log}(x^2+1) in O(mathrm{max}(x^2,xmathrm{log}(x^2+1)) = O(x^2) tag{4}$$



Any problems! Thanks!










share|cite|improve this question











$endgroup$




I wanted to know if the following solution demonstrates that the function $f(x) = (x+1)mathrm{log}, (x^2+1) + 3x^2 in O(x^2)$, because my answer and the book's answer deviate slightly.



Clearly,



$$3x^2 in O(x^2) tag{1}$$
$$x+1 in O(x)tag{2}$$



The following inequality is where the book and I differ, but I understand how the author obtained his big-O estimate. I said



$$mathrm{log}(x^2+1) in O(mathrm{log}(x^2+1))tag{3}$$



Therefore, the product of $(2)$ and $(3)$ renders



$$(x+1)mathrm{log}(x^2+1) in O(x mathrm{log}(x^2+1)) tag{3}$$



Finally, $(1)$ and $(3)$ gives us this big-O estimate



$$(x+1)mathrm{log}(x^2+1) in O(mathrm{max}(x^2,xmathrm{log}(x^2+1)) = O(x^2) tag{4}$$



Any problems! Thanks!







proof-verification asymptotics






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Feb 22 '16 at 4:36







Benedict Voltaire

















asked Feb 22 '16 at 4:07









Benedict VoltaireBenedict Voltaire

1,308928




1,308928












  • $begingroup$
    It's "obvious" to me (especially after looking at a couple of graphs on a graphing calculator) that $log(x^2 + 1) < x$ for all $x > 0$, but I don't know whether that's enough proof for your purposes. I'm also puzzled why you introduced the symbol $n$ in place of $1$.
    $endgroup$
    – David K
    Feb 22 '16 at 4:24












  • $begingroup$
    @DavidK Substituting $n$ for 1 was a mistake. And yes, it is clear that $mathrm{log}(x^2+1) in O(x)$, but I put $O(mathrm{log}(x^2+1)$ instead, because that came to me first.
    $endgroup$
    – Benedict Voltaire
    Feb 22 '16 at 4:37










  • $begingroup$
    Do you mean as $x to 0$ or $x to infty$?
    $endgroup$
    – marty cohen
    Feb 22 '16 at 4:59










  • $begingroup$
    I think you meant to write ${}+3x^2$ on the left of $(4)$, but aside from that it looks OK now. As noted in the answer by marty cohen, you could simplify the $O(log x)$ factor a bit, but in the end all you need is for that part to be $O(x)$. Let me guess, the book went directly to $O(x)$ for that part rather than waiting until line $(4)$, was that the difference in their approach?
    $endgroup$
    – David K
    Feb 22 '16 at 13:29










  • $begingroup$
    Actually, the book has $mathrm{log}(x^2+1) in O(mathrm{log}(x))$ in substitution for my $(3)$. Is it more natural to adopt a slower function as the author does when dealing with big-O estimates, because that's the only reason I can think of for the author not using $mathrm{log}(x^2+1) in O(mathrm{log}(x^2+1))$.
    $endgroup$
    – Benedict Voltaire
    Feb 22 '16 at 16:30




















  • $begingroup$
    It's "obvious" to me (especially after looking at a couple of graphs on a graphing calculator) that $log(x^2 + 1) < x$ for all $x > 0$, but I don't know whether that's enough proof for your purposes. I'm also puzzled why you introduced the symbol $n$ in place of $1$.
    $endgroup$
    – David K
    Feb 22 '16 at 4:24












  • $begingroup$
    @DavidK Substituting $n$ for 1 was a mistake. And yes, it is clear that $mathrm{log}(x^2+1) in O(x)$, but I put $O(mathrm{log}(x^2+1)$ instead, because that came to me first.
    $endgroup$
    – Benedict Voltaire
    Feb 22 '16 at 4:37










  • $begingroup$
    Do you mean as $x to 0$ or $x to infty$?
    $endgroup$
    – marty cohen
    Feb 22 '16 at 4:59










  • $begingroup$
    I think you meant to write ${}+3x^2$ on the left of $(4)$, but aside from that it looks OK now. As noted in the answer by marty cohen, you could simplify the $O(log x)$ factor a bit, but in the end all you need is for that part to be $O(x)$. Let me guess, the book went directly to $O(x)$ for that part rather than waiting until line $(4)$, was that the difference in their approach?
    $endgroup$
    – David K
    Feb 22 '16 at 13:29










  • $begingroup$
    Actually, the book has $mathrm{log}(x^2+1) in O(mathrm{log}(x))$ in substitution for my $(3)$. Is it more natural to adopt a slower function as the author does when dealing with big-O estimates, because that's the only reason I can think of for the author not using $mathrm{log}(x^2+1) in O(mathrm{log}(x^2+1))$.
    $endgroup$
    – Benedict Voltaire
    Feb 22 '16 at 16:30


















$begingroup$
It's "obvious" to me (especially after looking at a couple of graphs on a graphing calculator) that $log(x^2 + 1) < x$ for all $x > 0$, but I don't know whether that's enough proof for your purposes. I'm also puzzled why you introduced the symbol $n$ in place of $1$.
$endgroup$
– David K
Feb 22 '16 at 4:24






$begingroup$
It's "obvious" to me (especially after looking at a couple of graphs on a graphing calculator) that $log(x^2 + 1) < x$ for all $x > 0$, but I don't know whether that's enough proof for your purposes. I'm also puzzled why you introduced the symbol $n$ in place of $1$.
$endgroup$
– David K
Feb 22 '16 at 4:24














$begingroup$
@DavidK Substituting $n$ for 1 was a mistake. And yes, it is clear that $mathrm{log}(x^2+1) in O(x)$, but I put $O(mathrm{log}(x^2+1)$ instead, because that came to me first.
$endgroup$
– Benedict Voltaire
Feb 22 '16 at 4:37




$begingroup$
@DavidK Substituting $n$ for 1 was a mistake. And yes, it is clear that $mathrm{log}(x^2+1) in O(x)$, but I put $O(mathrm{log}(x^2+1)$ instead, because that came to me first.
$endgroup$
– Benedict Voltaire
Feb 22 '16 at 4:37












$begingroup$
Do you mean as $x to 0$ or $x to infty$?
$endgroup$
– marty cohen
Feb 22 '16 at 4:59




$begingroup$
Do you mean as $x to 0$ or $x to infty$?
$endgroup$
– marty cohen
Feb 22 '16 at 4:59












$begingroup$
I think you meant to write ${}+3x^2$ on the left of $(4)$, but aside from that it looks OK now. As noted in the answer by marty cohen, you could simplify the $O(log x)$ factor a bit, but in the end all you need is for that part to be $O(x)$. Let me guess, the book went directly to $O(x)$ for that part rather than waiting until line $(4)$, was that the difference in their approach?
$endgroup$
– David K
Feb 22 '16 at 13:29




$begingroup$
I think you meant to write ${}+3x^2$ on the left of $(4)$, but aside from that it looks OK now. As noted in the answer by marty cohen, you could simplify the $O(log x)$ factor a bit, but in the end all you need is for that part to be $O(x)$. Let me guess, the book went directly to $O(x)$ for that part rather than waiting until line $(4)$, was that the difference in their approach?
$endgroup$
– David K
Feb 22 '16 at 13:29












$begingroup$
Actually, the book has $mathrm{log}(x^2+1) in O(mathrm{log}(x))$ in substitution for my $(3)$. Is it more natural to adopt a slower function as the author does when dealing with big-O estimates, because that's the only reason I can think of for the author not using $mathrm{log}(x^2+1) in O(mathrm{log}(x^2+1))$.
$endgroup$
– Benedict Voltaire
Feb 22 '16 at 16:30






$begingroup$
Actually, the book has $mathrm{log}(x^2+1) in O(mathrm{log}(x))$ in substitution for my $(3)$. Is it more natural to adopt a slower function as the author does when dealing with big-O estimates, because that's the only reason I can think of for the author not using $mathrm{log}(x^2+1) in O(mathrm{log}(x^2+1))$.
$endgroup$
– Benedict Voltaire
Feb 22 '16 at 16:30












2 Answers
2






active

oldest

votes


















0












$begingroup$

If this is as
$x to infty$,
then
$log(x^k+a)
=O(log(x))
$
for any fixed $k$
and $a$.



Also,
$log(x)
=O(x^c)
$
for
any $c > 0$.



This should be enough.






share|cite|improve this answer









$endgroup$





















    0












    $begingroup$

    The issue from what I see is the in the $(x+1)mathrm{log}(x^2+1)$ portion of your proof. Here, you didn't properly derive the Big-O value to it's tightest bound. I'll give you a headstart on the problem and then I'll leave the rest for you.



    Within $mathrm{log}(x^2+1)$ you can note that $x^2 + 1 leq 2x^2$ whenever $x gt 1$. This is because if $x gt 1$ then $x^2 gt 1$, so we would just substitute $x^2$ for $1$ to obtain $x^2 + 1 leq 2x^2$. Now, given this information you can $mathrm{log}$ both sides and find the correct value, which should be $O(xmathrm{log}x)$.



    In general, it is always better to have your Big-O and Big-Omega notation as tight as possible so that you can most accurately determine the growth rate.



    Hope this helps.






    share|cite|improve this answer











    $endgroup$













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      2 Answers
      2






      active

      oldest

      votes








      2 Answers
      2






      active

      oldest

      votes









      active

      oldest

      votes






      active

      oldest

      votes









      0












      $begingroup$

      If this is as
      $x to infty$,
      then
      $log(x^k+a)
      =O(log(x))
      $
      for any fixed $k$
      and $a$.



      Also,
      $log(x)
      =O(x^c)
      $
      for
      any $c > 0$.



      This should be enough.






      share|cite|improve this answer









      $endgroup$


















        0












        $begingroup$

        If this is as
        $x to infty$,
        then
        $log(x^k+a)
        =O(log(x))
        $
        for any fixed $k$
        and $a$.



        Also,
        $log(x)
        =O(x^c)
        $
        for
        any $c > 0$.



        This should be enough.






        share|cite|improve this answer









        $endgroup$
















          0












          0








          0





          $begingroup$

          If this is as
          $x to infty$,
          then
          $log(x^k+a)
          =O(log(x))
          $
          for any fixed $k$
          and $a$.



          Also,
          $log(x)
          =O(x^c)
          $
          for
          any $c > 0$.



          This should be enough.






          share|cite|improve this answer









          $endgroup$



          If this is as
          $x to infty$,
          then
          $log(x^k+a)
          =O(log(x))
          $
          for any fixed $k$
          and $a$.



          Also,
          $log(x)
          =O(x^c)
          $
          for
          any $c > 0$.



          This should be enough.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Feb 22 '16 at 5:02









          marty cohenmarty cohen

          74k549128




          74k549128























              0












              $begingroup$

              The issue from what I see is the in the $(x+1)mathrm{log}(x^2+1)$ portion of your proof. Here, you didn't properly derive the Big-O value to it's tightest bound. I'll give you a headstart on the problem and then I'll leave the rest for you.



              Within $mathrm{log}(x^2+1)$ you can note that $x^2 + 1 leq 2x^2$ whenever $x gt 1$. This is because if $x gt 1$ then $x^2 gt 1$, so we would just substitute $x^2$ for $1$ to obtain $x^2 + 1 leq 2x^2$. Now, given this information you can $mathrm{log}$ both sides and find the correct value, which should be $O(xmathrm{log}x)$.



              In general, it is always better to have your Big-O and Big-Omega notation as tight as possible so that you can most accurately determine the growth rate.



              Hope this helps.






              share|cite|improve this answer











              $endgroup$


















                0












                $begingroup$

                The issue from what I see is the in the $(x+1)mathrm{log}(x^2+1)$ portion of your proof. Here, you didn't properly derive the Big-O value to it's tightest bound. I'll give you a headstart on the problem and then I'll leave the rest for you.



                Within $mathrm{log}(x^2+1)$ you can note that $x^2 + 1 leq 2x^2$ whenever $x gt 1$. This is because if $x gt 1$ then $x^2 gt 1$, so we would just substitute $x^2$ for $1$ to obtain $x^2 + 1 leq 2x^2$. Now, given this information you can $mathrm{log}$ both sides and find the correct value, which should be $O(xmathrm{log}x)$.



                In general, it is always better to have your Big-O and Big-Omega notation as tight as possible so that you can most accurately determine the growth rate.



                Hope this helps.






                share|cite|improve this answer











                $endgroup$
















                  0












                  0








                  0





                  $begingroup$

                  The issue from what I see is the in the $(x+1)mathrm{log}(x^2+1)$ portion of your proof. Here, you didn't properly derive the Big-O value to it's tightest bound. I'll give you a headstart on the problem and then I'll leave the rest for you.



                  Within $mathrm{log}(x^2+1)$ you can note that $x^2 + 1 leq 2x^2$ whenever $x gt 1$. This is because if $x gt 1$ then $x^2 gt 1$, so we would just substitute $x^2$ for $1$ to obtain $x^2 + 1 leq 2x^2$. Now, given this information you can $mathrm{log}$ both sides and find the correct value, which should be $O(xmathrm{log}x)$.



                  In general, it is always better to have your Big-O and Big-Omega notation as tight as possible so that you can most accurately determine the growth rate.



                  Hope this helps.






                  share|cite|improve this answer











                  $endgroup$



                  The issue from what I see is the in the $(x+1)mathrm{log}(x^2+1)$ portion of your proof. Here, you didn't properly derive the Big-O value to it's tightest bound. I'll give you a headstart on the problem and then I'll leave the rest for you.



                  Within $mathrm{log}(x^2+1)$ you can note that $x^2 + 1 leq 2x^2$ whenever $x gt 1$. This is because if $x gt 1$ then $x^2 gt 1$, so we would just substitute $x^2$ for $1$ to obtain $x^2 + 1 leq 2x^2$. Now, given this information you can $mathrm{log}$ both sides and find the correct value, which should be $O(xmathrm{log}x)$.



                  In general, it is always better to have your Big-O and Big-Omega notation as tight as possible so that you can most accurately determine the growth rate.



                  Hope this helps.







                  share|cite|improve this answer














                  share|cite|improve this answer



                  share|cite|improve this answer








                  edited Apr 19 '18 at 21:53

























                  answered Apr 19 '18 at 21:30









                  PrinceOfPersiaPrinceOfPersia

                  114




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