Proving $f(x)=|x|$ is onto












2












$begingroup$


I've been working on proving that this is a onto function:



$f$ : $mathbb R$ $to$ $mathbb R^{geq0}$ is defined by $f(x)=|x|$



My proof so far: Let $yinmathbb R$.



Rough work: $|x|=y Rightarrow sqrt {x^2}=y Rightarrow n^2=y^2 Rightarrow pm x=pm y$



Suppose $f(pm y)=|pm y|=y$.



I know that this function is definitely onto given the co-domain of $mathbb R^{geq0}$, but I feel like my proof is flawed. Am I supposed to individually account for the $-x$ and the $+x$ from $pm x=pm y$ when trying to solve $f(x) = y$?



Thanks!










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$endgroup$








  • 1




    $begingroup$
    Let $ain mathbb R ^{geq 0}$. Then by definition, $|a|=a$. Therefore, $a$ is in the image of $f:xmapsto |x|$.
    $endgroup$
    – Arrow
    yesterday






  • 6




    $begingroup$
    You proof is wrong from the beginning. You need to assume y is in the nonnegative reals and show you can find a real x that maps to it.
    $endgroup$
    – symplectomorphic
    yesterday
















2












$begingroup$


I've been working on proving that this is a onto function:



$f$ : $mathbb R$ $to$ $mathbb R^{geq0}$ is defined by $f(x)=|x|$



My proof so far: Let $yinmathbb R$.



Rough work: $|x|=y Rightarrow sqrt {x^2}=y Rightarrow n^2=y^2 Rightarrow pm x=pm y$



Suppose $f(pm y)=|pm y|=y$.



I know that this function is definitely onto given the co-domain of $mathbb R^{geq0}$, but I feel like my proof is flawed. Am I supposed to individually account for the $-x$ and the $+x$ from $pm x=pm y$ when trying to solve $f(x) = y$?



Thanks!










share|cite|improve this question











$endgroup$








  • 1




    $begingroup$
    Let $ain mathbb R ^{geq 0}$. Then by definition, $|a|=a$. Therefore, $a$ is in the image of $f:xmapsto |x|$.
    $endgroup$
    – Arrow
    yesterday






  • 6




    $begingroup$
    You proof is wrong from the beginning. You need to assume y is in the nonnegative reals and show you can find a real x that maps to it.
    $endgroup$
    – symplectomorphic
    yesterday














2












2








2


2



$begingroup$


I've been working on proving that this is a onto function:



$f$ : $mathbb R$ $to$ $mathbb R^{geq0}$ is defined by $f(x)=|x|$



My proof so far: Let $yinmathbb R$.



Rough work: $|x|=y Rightarrow sqrt {x^2}=y Rightarrow n^2=y^2 Rightarrow pm x=pm y$



Suppose $f(pm y)=|pm y|=y$.



I know that this function is definitely onto given the co-domain of $mathbb R^{geq0}$, but I feel like my proof is flawed. Am I supposed to individually account for the $-x$ and the $+x$ from $pm x=pm y$ when trying to solve $f(x) = y$?



Thanks!










share|cite|improve this question











$endgroup$




I've been working on proving that this is a onto function:



$f$ : $mathbb R$ $to$ $mathbb R^{geq0}$ is defined by $f(x)=|x|$



My proof so far: Let $yinmathbb R$.



Rough work: $|x|=y Rightarrow sqrt {x^2}=y Rightarrow n^2=y^2 Rightarrow pm x=pm y$



Suppose $f(pm y)=|pm y|=y$.



I know that this function is definitely onto given the co-domain of $mathbb R^{geq0}$, but I feel like my proof is flawed. Am I supposed to individually account for the $-x$ and the $+x$ from $pm x=pm y$ when trying to solve $f(x) = y$?



Thanks!







proof-verification elementary-set-theory






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edited 21 hours ago









YuiTo Cheng

2,1362837




2,1362837










asked yesterday









Nick SabiaNick Sabia

285




285








  • 1




    $begingroup$
    Let $ain mathbb R ^{geq 0}$. Then by definition, $|a|=a$. Therefore, $a$ is in the image of $f:xmapsto |x|$.
    $endgroup$
    – Arrow
    yesterday






  • 6




    $begingroup$
    You proof is wrong from the beginning. You need to assume y is in the nonnegative reals and show you can find a real x that maps to it.
    $endgroup$
    – symplectomorphic
    yesterday














  • 1




    $begingroup$
    Let $ain mathbb R ^{geq 0}$. Then by definition, $|a|=a$. Therefore, $a$ is in the image of $f:xmapsto |x|$.
    $endgroup$
    – Arrow
    yesterday






  • 6




    $begingroup$
    You proof is wrong from the beginning. You need to assume y is in the nonnegative reals and show you can find a real x that maps to it.
    $endgroup$
    – symplectomorphic
    yesterday








1




1




$begingroup$
Let $ain mathbb R ^{geq 0}$. Then by definition, $|a|=a$. Therefore, $a$ is in the image of $f:xmapsto |x|$.
$endgroup$
– Arrow
yesterday




$begingroup$
Let $ain mathbb R ^{geq 0}$. Then by definition, $|a|=a$. Therefore, $a$ is in the image of $f:xmapsto |x|$.
$endgroup$
– Arrow
yesterday




6




6




$begingroup$
You proof is wrong from the beginning. You need to assume y is in the nonnegative reals and show you can find a real x that maps to it.
$endgroup$
– symplectomorphic
yesterday




$begingroup$
You proof is wrong from the beginning. You need to assume y is in the nonnegative reals and show you can find a real x that maps to it.
$endgroup$
– symplectomorphic
yesterday










5 Answers
5






active

oldest

votes


















21












$begingroup$

In order to show that the function is onto (surjective) it is enough to argue that for each $y$ in the codomain there is at least one $x$ in the domain that maps to it.



You seem to be trying to find all of the $x$ such that $f(x)=y$, which is more work than you need to do and creates a rather large detour.



You could just say:




Let $yinmathbb R^{ge 0}$ be given. Then $f(y)=|y|=y$, so by setting $x=y$ we find an $xinmathbb R$ such that $f(x)=y$. Since $y$ was arbitrary this proves that $f$ is surjective.




(Even this is more verbose than it really needs to be, but in exercises at this elementary level it is good to be explicit about the details, to show that you know what you're doing).






share|cite|improve this answer









$endgroup$









  • 1




    $begingroup$
    (+1) Because this is a vey pedagogical answer.
    $endgroup$
    – José Carlos Santos
    yesterday



















4












$begingroup$


Am I supposed to individually account for the -x and the +x from ±x=±y when trying to solve f(x)=y?




No.



You are simply supposed to show that for any general arbitrary $y in mathbb R^{ge 0}$ that there is, at least (you don't have to find them all), one $xin mathbb R$ so that $|x| = y$.



As $|y| = y$ this is very easy. And you are done.



The proof is two lines:



1) Let $y in mathbb R^{ge 0}$.



2) $f(y) = |y| = y$.






share|cite|improve this answer









$endgroup$





















    2












    $begingroup$

    You should not start with $yinmathbb R$ but rather with $yinmathbb{R}^{geqslant0}$. Then $y=f(y)$. Since this happens for every $yinmathbb{R}^{geqslant 0}$, $f$ is onto.






    share|cite|improve this answer











    $endgroup$





















      0












      $begingroup$

      Well, you want to show that $f$ is onto. So take an arbitrary element $yin{Bbb R}_{geq 0}$ from the image set and find a preimage. Here a preimage is $y$ itself or even $-y$, since $|-y|=y$.






      share|cite|improve this answer









      $endgroup$





















        0












        $begingroup$

        More generally, If $B subseteq A$ and $f: A rightarrow B$ is a continuous mapping with $f(b) = b$ for all $b in B$ then $f$ is called a retraction of $A$ onto $B$. It is trivial that any such $f$ is onto since $f(b) = b$ for all $b in B$ immediately implies that all such $b$ are in the range. Note that continuity really plays no role in this. Thinking of $|x|$ as a retraction is perhaps overkill, but shows that the proof that it is onto the nonnegative reals is a special case of a more general (and equally easy to prove) result.






        share|cite|improve this answer









        $endgroup$













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          5 Answers
          5






          active

          oldest

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          5 Answers
          5






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          21












          $begingroup$

          In order to show that the function is onto (surjective) it is enough to argue that for each $y$ in the codomain there is at least one $x$ in the domain that maps to it.



          You seem to be trying to find all of the $x$ such that $f(x)=y$, which is more work than you need to do and creates a rather large detour.



          You could just say:




          Let $yinmathbb R^{ge 0}$ be given. Then $f(y)=|y|=y$, so by setting $x=y$ we find an $xinmathbb R$ such that $f(x)=y$. Since $y$ was arbitrary this proves that $f$ is surjective.




          (Even this is more verbose than it really needs to be, but in exercises at this elementary level it is good to be explicit about the details, to show that you know what you're doing).






          share|cite|improve this answer









          $endgroup$









          • 1




            $begingroup$
            (+1) Because this is a vey pedagogical answer.
            $endgroup$
            – José Carlos Santos
            yesterday
















          21












          $begingroup$

          In order to show that the function is onto (surjective) it is enough to argue that for each $y$ in the codomain there is at least one $x$ in the domain that maps to it.



          You seem to be trying to find all of the $x$ such that $f(x)=y$, which is more work than you need to do and creates a rather large detour.



          You could just say:




          Let $yinmathbb R^{ge 0}$ be given. Then $f(y)=|y|=y$, so by setting $x=y$ we find an $xinmathbb R$ such that $f(x)=y$. Since $y$ was arbitrary this proves that $f$ is surjective.




          (Even this is more verbose than it really needs to be, but in exercises at this elementary level it is good to be explicit about the details, to show that you know what you're doing).






          share|cite|improve this answer









          $endgroup$









          • 1




            $begingroup$
            (+1) Because this is a vey pedagogical answer.
            $endgroup$
            – José Carlos Santos
            yesterday














          21












          21








          21





          $begingroup$

          In order to show that the function is onto (surjective) it is enough to argue that for each $y$ in the codomain there is at least one $x$ in the domain that maps to it.



          You seem to be trying to find all of the $x$ such that $f(x)=y$, which is more work than you need to do and creates a rather large detour.



          You could just say:




          Let $yinmathbb R^{ge 0}$ be given. Then $f(y)=|y|=y$, so by setting $x=y$ we find an $xinmathbb R$ such that $f(x)=y$. Since $y$ was arbitrary this proves that $f$ is surjective.




          (Even this is more verbose than it really needs to be, but in exercises at this elementary level it is good to be explicit about the details, to show that you know what you're doing).






          share|cite|improve this answer









          $endgroup$



          In order to show that the function is onto (surjective) it is enough to argue that for each $y$ in the codomain there is at least one $x$ in the domain that maps to it.



          You seem to be trying to find all of the $x$ such that $f(x)=y$, which is more work than you need to do and creates a rather large detour.



          You could just say:




          Let $yinmathbb R^{ge 0}$ be given. Then $f(y)=|y|=y$, so by setting $x=y$ we find an $xinmathbb R$ such that $f(x)=y$. Since $y$ was arbitrary this proves that $f$ is surjective.




          (Even this is more verbose than it really needs to be, but in exercises at this elementary level it is good to be explicit about the details, to show that you know what you're doing).







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered yesterday









          Henning MakholmHenning Makholm

          242k17308551




          242k17308551








          • 1




            $begingroup$
            (+1) Because this is a vey pedagogical answer.
            $endgroup$
            – José Carlos Santos
            yesterday














          • 1




            $begingroup$
            (+1) Because this is a vey pedagogical answer.
            $endgroup$
            – José Carlos Santos
            yesterday








          1




          1




          $begingroup$
          (+1) Because this is a vey pedagogical answer.
          $endgroup$
          – José Carlos Santos
          yesterday




          $begingroup$
          (+1) Because this is a vey pedagogical answer.
          $endgroup$
          – José Carlos Santos
          yesterday











          4












          $begingroup$


          Am I supposed to individually account for the -x and the +x from ±x=±y when trying to solve f(x)=y?




          No.



          You are simply supposed to show that for any general arbitrary $y in mathbb R^{ge 0}$ that there is, at least (you don't have to find them all), one $xin mathbb R$ so that $|x| = y$.



          As $|y| = y$ this is very easy. And you are done.



          The proof is two lines:



          1) Let $y in mathbb R^{ge 0}$.



          2) $f(y) = |y| = y$.






          share|cite|improve this answer









          $endgroup$


















            4












            $begingroup$


            Am I supposed to individually account for the -x and the +x from ±x=±y when trying to solve f(x)=y?




            No.



            You are simply supposed to show that for any general arbitrary $y in mathbb R^{ge 0}$ that there is, at least (you don't have to find them all), one $xin mathbb R$ so that $|x| = y$.



            As $|y| = y$ this is very easy. And you are done.



            The proof is two lines:



            1) Let $y in mathbb R^{ge 0}$.



            2) $f(y) = |y| = y$.






            share|cite|improve this answer









            $endgroup$
















              4












              4








              4





              $begingroup$


              Am I supposed to individually account for the -x and the +x from ±x=±y when trying to solve f(x)=y?




              No.



              You are simply supposed to show that for any general arbitrary $y in mathbb R^{ge 0}$ that there is, at least (you don't have to find them all), one $xin mathbb R$ so that $|x| = y$.



              As $|y| = y$ this is very easy. And you are done.



              The proof is two lines:



              1) Let $y in mathbb R^{ge 0}$.



              2) $f(y) = |y| = y$.






              share|cite|improve this answer









              $endgroup$




              Am I supposed to individually account for the -x and the +x from ±x=±y when trying to solve f(x)=y?




              No.



              You are simply supposed to show that for any general arbitrary $y in mathbb R^{ge 0}$ that there is, at least (you don't have to find them all), one $xin mathbb R$ so that $|x| = y$.



              As $|y| = y$ this is very easy. And you are done.



              The proof is two lines:



              1) Let $y in mathbb R^{ge 0}$.



              2) $f(y) = |y| = y$.







              share|cite|improve this answer












              share|cite|improve this answer



              share|cite|improve this answer










              answered yesterday









              fleabloodfleablood

              73.4k22791




              73.4k22791























                  2












                  $begingroup$

                  You should not start with $yinmathbb R$ but rather with $yinmathbb{R}^{geqslant0}$. Then $y=f(y)$. Since this happens for every $yinmathbb{R}^{geqslant 0}$, $f$ is onto.






                  share|cite|improve this answer











                  $endgroup$


















                    2












                    $begingroup$

                    You should not start with $yinmathbb R$ but rather with $yinmathbb{R}^{geqslant0}$. Then $y=f(y)$. Since this happens for every $yinmathbb{R}^{geqslant 0}$, $f$ is onto.






                    share|cite|improve this answer











                    $endgroup$
















                      2












                      2








                      2





                      $begingroup$

                      You should not start with $yinmathbb R$ but rather with $yinmathbb{R}^{geqslant0}$. Then $y=f(y)$. Since this happens for every $yinmathbb{R}^{geqslant 0}$, $f$ is onto.






                      share|cite|improve this answer











                      $endgroup$



                      You should not start with $yinmathbb R$ but rather with $yinmathbb{R}^{geqslant0}$. Then $y=f(y)$. Since this happens for every $yinmathbb{R}^{geqslant 0}$, $f$ is onto.







                      share|cite|improve this answer














                      share|cite|improve this answer



                      share|cite|improve this answer








                      edited yesterday









                      J. W. Tanner

                      3,8001320




                      3,8001320










                      answered yesterday









                      José Carlos SantosJosé Carlos Santos

                      170k23132238




                      170k23132238























                          0












                          $begingroup$

                          Well, you want to show that $f$ is onto. So take an arbitrary element $yin{Bbb R}_{geq 0}$ from the image set and find a preimage. Here a preimage is $y$ itself or even $-y$, since $|-y|=y$.






                          share|cite|improve this answer









                          $endgroup$


















                            0












                            $begingroup$

                            Well, you want to show that $f$ is onto. So take an arbitrary element $yin{Bbb R}_{geq 0}$ from the image set and find a preimage. Here a preimage is $y$ itself or even $-y$, since $|-y|=y$.






                            share|cite|improve this answer









                            $endgroup$
















                              0












                              0








                              0





                              $begingroup$

                              Well, you want to show that $f$ is onto. So take an arbitrary element $yin{Bbb R}_{geq 0}$ from the image set and find a preimage. Here a preimage is $y$ itself or even $-y$, since $|-y|=y$.






                              share|cite|improve this answer









                              $endgroup$



                              Well, you want to show that $f$ is onto. So take an arbitrary element $yin{Bbb R}_{geq 0}$ from the image set and find a preimage. Here a preimage is $y$ itself or even $-y$, since $|-y|=y$.







                              share|cite|improve this answer












                              share|cite|improve this answer



                              share|cite|improve this answer










                              answered yesterday









                              WuestenfuxWuestenfux

                              5,2981513




                              5,2981513























                                  0












                                  $begingroup$

                                  More generally, If $B subseteq A$ and $f: A rightarrow B$ is a continuous mapping with $f(b) = b$ for all $b in B$ then $f$ is called a retraction of $A$ onto $B$. It is trivial that any such $f$ is onto since $f(b) = b$ for all $b in B$ immediately implies that all such $b$ are in the range. Note that continuity really plays no role in this. Thinking of $|x|$ as a retraction is perhaps overkill, but shows that the proof that it is onto the nonnegative reals is a special case of a more general (and equally easy to prove) result.






                                  share|cite|improve this answer









                                  $endgroup$


















                                    0












                                    $begingroup$

                                    More generally, If $B subseteq A$ and $f: A rightarrow B$ is a continuous mapping with $f(b) = b$ for all $b in B$ then $f$ is called a retraction of $A$ onto $B$. It is trivial that any such $f$ is onto since $f(b) = b$ for all $b in B$ immediately implies that all such $b$ are in the range. Note that continuity really plays no role in this. Thinking of $|x|$ as a retraction is perhaps overkill, but shows that the proof that it is onto the nonnegative reals is a special case of a more general (and equally easy to prove) result.






                                    share|cite|improve this answer









                                    $endgroup$
















                                      0












                                      0








                                      0





                                      $begingroup$

                                      More generally, If $B subseteq A$ and $f: A rightarrow B$ is a continuous mapping with $f(b) = b$ for all $b in B$ then $f$ is called a retraction of $A$ onto $B$. It is trivial that any such $f$ is onto since $f(b) = b$ for all $b in B$ immediately implies that all such $b$ are in the range. Note that continuity really plays no role in this. Thinking of $|x|$ as a retraction is perhaps overkill, but shows that the proof that it is onto the nonnegative reals is a special case of a more general (and equally easy to prove) result.






                                      share|cite|improve this answer









                                      $endgroup$



                                      More generally, If $B subseteq A$ and $f: A rightarrow B$ is a continuous mapping with $f(b) = b$ for all $b in B$ then $f$ is called a retraction of $A$ onto $B$. It is trivial that any such $f$ is onto since $f(b) = b$ for all $b in B$ immediately implies that all such $b$ are in the range. Note that continuity really plays no role in this. Thinking of $|x|$ as a retraction is perhaps overkill, but shows that the proof that it is onto the nonnegative reals is a special case of a more general (and equally easy to prove) result.







                                      share|cite|improve this answer












                                      share|cite|improve this answer



                                      share|cite|improve this answer










                                      answered yesterday









                                      John ColemanJohn Coleman

                                      3,98311224




                                      3,98311224






























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                                          Старые Смолеговицы Содержание История | География | Демография | Достопримечательности | Примечания | НавигацияHGЯOLHGЯOL41 206 832 01641 606 406 141Административно-территориальное деление Ленинградской области«Переписная оброчная книга Водской пятины 1500 года», С. 793«Карта Ингерманландии: Ивангорода, Яма, Копорья, Нотеборга», по материалам 1676 г.«Генеральная карта провинции Ингерманландии» Э. Белинга и А. Андерсина, 1704 г., составлена по материалам 1678 г.«Географический чертёж над Ижорскою землей со своими городами» Адриана Шонбека 1705 г.Новая и достоверная всей Ингерманландии ланткарта. Грав. А. Ростовцев. СПб., 1727 г.Топографическая карта Санкт-Петербургской губернии. 5-и верстка. Шуберт. 1834 г.Описание Санкт-Петербургской губернии по уездам и станамСпецкарта западной части России Ф. Ф. Шуберта. 1844 г.Алфавитный список селений по уездам и станам С.-Петербургской губернииСписки населённых мест Российской Империи, составленные и издаваемые центральным статистическим комитетом министерства внутренних дел. XXXVII. Санкт-Петербургская губерния. По состоянию на 1862 год. СПб. 1864. С. 203Материалы по статистике народного хозяйства в С.-Петербургской губернии. Вып. IX. Частновладельческое хозяйство в Ямбургском уезде. СПб, 1888, С. 146, С. 2, 7, 54Положение о гербе муниципального образования Курское сельское поселениеСправочник истории административно-территориального деления Ленинградской области.Топографическая карта Ленинградской области, квадрат О-35-23-В (Хотыницы), 1930 г.АрхивированоАдминистративно-территориальное деление Ленинградской области. — Л., 1933, С. 27, 198АрхивированоАдминистративно-экономический справочник по Ленинградской области. — Л., 1936, с. 219АрхивированоАдминистративно-территориальное деление Ленинградской области. — Л., 1966, с. 175АрхивированоАдминистративно-территориальное деление Ленинградской области. — Лениздат, 1973, С. 180АрхивированоАдминистративно-территориальное деление Ленинградской области. — Лениздат, 1990, ISBN 5-289-00612-5, С. 38АрхивированоАдминистративно-территориальное деление Ленинградской области. — СПб., 2007, с. 60АрхивированоКоряков Юрий База данных «Этно-языковой состав населённых пунктов России». Ленинградская область.Административно-территориальное деление Ленинградской области. — СПб, 1997, ISBN 5-86153-055-6, С. 41АрхивированоКультовый комплекс Старые Смолеговицы // Электронная энциклопедия ЭрмитажаПроблемы выявления, изучения и сохранения культовых комплексов с каменными крестами: по материалам работ 2016-2017 гг. в Ленинградской области