Equality of fractions in localization as quotient












2












$begingroup$


Let $Ssubset A$ be a subset of a commutative ring. Consider the polynomial algebra $A[x_smid sin S]$ and define the localization $A_S$ as the quotient by the ideal $ (sx_s-1mid sin S)vartriangleleft A[x_smid sin S]$.



Let us write $frac as$ for the equivalence class of $ax_s$ in the quotient $A_S$. I would like to prove that $frac as=frac bt$ iff $exists uin S$ such that $u(at-bs)=0$, like in the construction of localization as a quotient of $Atimes S$.



So we have $frac as-frac bt=0in A_S$ which is equivalent to $ax_s-bx_tin I$. By degree considerations I think we must have $ax_s-bx_t=c(sx_s-1)-c(tx_t-1)$ for some $cin A$, and by comparing coefficients we have $a=cs,b=ct$. Multiplying by $t,s$ respectively we find $at=cst=cts=bs$, whence $at-bs=0$ without any $uin S$.



What is my mistake?










share|cite|improve this question









$endgroup$

















    2












    $begingroup$


    Let $Ssubset A$ be a subset of a commutative ring. Consider the polynomial algebra $A[x_smid sin S]$ and define the localization $A_S$ as the quotient by the ideal $ (sx_s-1mid sin S)vartriangleleft A[x_smid sin S]$.



    Let us write $frac as$ for the equivalence class of $ax_s$ in the quotient $A_S$. I would like to prove that $frac as=frac bt$ iff $exists uin S$ such that $u(at-bs)=0$, like in the construction of localization as a quotient of $Atimes S$.



    So we have $frac as-frac bt=0in A_S$ which is equivalent to $ax_s-bx_tin I$. By degree considerations I think we must have $ax_s-bx_t=c(sx_s-1)-c(tx_t-1)$ for some $cin A$, and by comparing coefficients we have $a=cs,b=ct$. Multiplying by $t,s$ respectively we find $at=cst=cts=bs$, whence $at-bs=0$ without any $uin S$.



    What is my mistake?










    share|cite|improve this question









    $endgroup$















      2












      2








      2





      $begingroup$


      Let $Ssubset A$ be a subset of a commutative ring. Consider the polynomial algebra $A[x_smid sin S]$ and define the localization $A_S$ as the quotient by the ideal $ (sx_s-1mid sin S)vartriangleleft A[x_smid sin S]$.



      Let us write $frac as$ for the equivalence class of $ax_s$ in the quotient $A_S$. I would like to prove that $frac as=frac bt$ iff $exists uin S$ such that $u(at-bs)=0$, like in the construction of localization as a quotient of $Atimes S$.



      So we have $frac as-frac bt=0in A_S$ which is equivalent to $ax_s-bx_tin I$. By degree considerations I think we must have $ax_s-bx_t=c(sx_s-1)-c(tx_t-1)$ for some $cin A$, and by comparing coefficients we have $a=cs,b=ct$. Multiplying by $t,s$ respectively we find $at=cst=cts=bs$, whence $at-bs=0$ without any $uin S$.



      What is my mistake?










      share|cite|improve this question









      $endgroup$




      Let $Ssubset A$ be a subset of a commutative ring. Consider the polynomial algebra $A[x_smid sin S]$ and define the localization $A_S$ as the quotient by the ideal $ (sx_s-1mid sin S)vartriangleleft A[x_smid sin S]$.



      Let us write $frac as$ for the equivalence class of $ax_s$ in the quotient $A_S$. I would like to prove that $frac as=frac bt$ iff $exists uin S$ such that $u(at-bs)=0$, like in the construction of localization as a quotient of $Atimes S$.



      So we have $frac as-frac bt=0in A_S$ which is equivalent to $ax_s-bx_tin I$. By degree considerations I think we must have $ax_s-bx_t=c(sx_s-1)-c(tx_t-1)$ for some $cin A$, and by comparing coefficients we have $a=cs,b=ct$. Multiplying by $t,s$ respectively we find $at=cst=cts=bs$, whence $at-bs=0$ without any $uin S$.



      What is my mistake?







      commutative-algebra localization






      share|cite|improve this question













      share|cite|improve this question











      share|cite|improve this question




      share|cite|improve this question










      asked Feb 1 at 0:38









      ArrowArrow

      5,20831546




      5,20831546






















          1 Answer
          1






          active

          oldest

          votes


















          1












          $begingroup$

          I think the main mistake is in the statement "by degree considerations..." I did not verify that it is true for integral domains, but these considerations are more complicated in the presence of zerodivisors.



          Let's consider a (classical) localization in a ring $A$ with zerodivisors. For example, let $A = mathbb{Z}[y]/(2y)$ and invert $2$. Then in the localization $y=0$.



          Now consider your polynomial-ring construction, in which $2x_2 - 1 = 0$ and (multiplying by $y$) $$2x_2 y - y = 0,$$ i.e. $y=0$. This looks like a separate, more minor, problem: it is a relation not of the form $$frac{a}{s}=frac{b}{t}$$ for $s,t in S$ (you might want to take $1 in S$.) I'll assume we took $1in S$, and use a corresponding variable $x_1$ below.



          Anyway, $y=0$ and in particular we have for example $$yx_2 - 0x_1 = yx_2 in I.$$ So by your degree considerations, we should get $yx_2 = c(2x_2 - 1) - c(1x_1-1)$ in $A[x_1, x_2]$ for some $cin A$. This is not the case. To exhibit this relation as an element of $I$ you need something like $yx_2 cdot (2x_2 - 1),$ where $yx_2$ is not of degree $0$, hence not in $A$, so the degree considerations fail.





          I previously wrote a "correction" to your proof, but it was mainly nonsense. There was some germ of a good idea hidden there, though. Here is a sketch of a correct proof:



          Consider a ring $A$ and a module $M$ over it, and take $$(Moplus M) / ((x,sx))$$ for some fixed $sin A$. What is the submodule generated by the first coordinate? If an element $x$ is annihilated by $s$, then $$(x,0) = (x,0) - (x,sx) = (0,0)$$ in the quotient. It can be verified directly that the submodule is just $$frac{M}{text{ker}(s:Mrightarrow M)}.$$ What if we take $$bigoplus_{i,j ge 0} M$$ and mod out by elements of the form $$se_{i+1,j}-e_{i,j},quad te_{i,j+1}-e_{i,j}?$$ I claim the $e_{0,0}$ coordinate then generates a module isomorphic to $$frac{M}{bigcup_n text{ker}(st)^n:Mrightarrow M},$$ et cetera.



          To see this, consider the colimit explicitly. If an element of the submodule generated by $M_{0,0}$ in the quotient vanishes, it vanishes already in some f.g. submodule, which is contained in some direct sum of a finite set of $M_{i,j}$, and hence in a "rectangular" submodule $$bigoplus_{0le i,jle n} M_{i,j}.$$ Any element here can be "pushed" to one supported only in $M_{n,n}$, and it is easy to check what it means that it vanishes there. The same argument generalizes to higher "dimensions" $$bigoplus M_{i,j,k}.$$



          You want to take $$B:= Aleft[{x_s}_{sin S}right]$$ and consider some ideal generated by elements of the form $(sx_s - 1)$. Considering the polynomial ring as a free $A$-module, the required statement follows from the previous one, once one shows the ideal is generated as an $A$-submodule of $B$ by elements of the same form as above. This is not so bad since $$(sx_s -1)(tx_t - 1) = (stx_sx_t - 1) - (sx_s -1) - (tx_t - 1)$$ is already a sum of three relations, and the same will occur for more complicated monomials in place of $x_s$ and $x_t$. Multiplying a relation by a monomial also gives you a sum of relations in the quotient submodule of $$bigoplus_{i,j}M_i,j$$ above.






          share|cite|improve this answer











          $endgroup$














            Your Answer





            StackExchange.ifUsing("editor", function () {
            return StackExchange.using("mathjaxEditing", function () {
            StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
            StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
            });
            });
            }, "mathjax-editing");

            StackExchange.ready(function() {
            var channelOptions = {
            tags: "".split(" "),
            id: "69"
            };
            initTagRenderer("".split(" "), "".split(" "), channelOptions);

            StackExchange.using("externalEditor", function() {
            // Have to fire editor after snippets, if snippets enabled
            if (StackExchange.settings.snippets.snippetsEnabled) {
            StackExchange.using("snippets", function() {
            createEditor();
            });
            }
            else {
            createEditor();
            }
            });

            function createEditor() {
            StackExchange.prepareEditor({
            heartbeatType: 'answer',
            autoActivateHeartbeat: false,
            convertImagesToLinks: true,
            noModals: true,
            showLowRepImageUploadWarning: true,
            reputationToPostImages: 10,
            bindNavPrevention: true,
            postfix: "",
            imageUploader: {
            brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
            contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
            allowUrls: true
            },
            noCode: true, onDemand: true,
            discardSelector: ".discard-answer"
            ,immediatelyShowMarkdownHelp:true
            });


            }
            });














            draft saved

            draft discarded


















            StackExchange.ready(
            function () {
            StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3095680%2fequality-of-fractions-in-localization-as-quotient%23new-answer', 'question_page');
            }
            );

            Post as a guest















            Required, but never shown

























            1 Answer
            1






            active

            oldest

            votes








            1 Answer
            1






            active

            oldest

            votes









            active

            oldest

            votes






            active

            oldest

            votes









            1












            $begingroup$

            I think the main mistake is in the statement "by degree considerations..." I did not verify that it is true for integral domains, but these considerations are more complicated in the presence of zerodivisors.



            Let's consider a (classical) localization in a ring $A$ with zerodivisors. For example, let $A = mathbb{Z}[y]/(2y)$ and invert $2$. Then in the localization $y=0$.



            Now consider your polynomial-ring construction, in which $2x_2 - 1 = 0$ and (multiplying by $y$) $$2x_2 y - y = 0,$$ i.e. $y=0$. This looks like a separate, more minor, problem: it is a relation not of the form $$frac{a}{s}=frac{b}{t}$$ for $s,t in S$ (you might want to take $1 in S$.) I'll assume we took $1in S$, and use a corresponding variable $x_1$ below.



            Anyway, $y=0$ and in particular we have for example $$yx_2 - 0x_1 = yx_2 in I.$$ So by your degree considerations, we should get $yx_2 = c(2x_2 - 1) - c(1x_1-1)$ in $A[x_1, x_2]$ for some $cin A$. This is not the case. To exhibit this relation as an element of $I$ you need something like $yx_2 cdot (2x_2 - 1),$ where $yx_2$ is not of degree $0$, hence not in $A$, so the degree considerations fail.





            I previously wrote a "correction" to your proof, but it was mainly nonsense. There was some germ of a good idea hidden there, though. Here is a sketch of a correct proof:



            Consider a ring $A$ and a module $M$ over it, and take $$(Moplus M) / ((x,sx))$$ for some fixed $sin A$. What is the submodule generated by the first coordinate? If an element $x$ is annihilated by $s$, then $$(x,0) = (x,0) - (x,sx) = (0,0)$$ in the quotient. It can be verified directly that the submodule is just $$frac{M}{text{ker}(s:Mrightarrow M)}.$$ What if we take $$bigoplus_{i,j ge 0} M$$ and mod out by elements of the form $$se_{i+1,j}-e_{i,j},quad te_{i,j+1}-e_{i,j}?$$ I claim the $e_{0,0}$ coordinate then generates a module isomorphic to $$frac{M}{bigcup_n text{ker}(st)^n:Mrightarrow M},$$ et cetera.



            To see this, consider the colimit explicitly. If an element of the submodule generated by $M_{0,0}$ in the quotient vanishes, it vanishes already in some f.g. submodule, which is contained in some direct sum of a finite set of $M_{i,j}$, and hence in a "rectangular" submodule $$bigoplus_{0le i,jle n} M_{i,j}.$$ Any element here can be "pushed" to one supported only in $M_{n,n}$, and it is easy to check what it means that it vanishes there. The same argument generalizes to higher "dimensions" $$bigoplus M_{i,j,k}.$$



            You want to take $$B:= Aleft[{x_s}_{sin S}right]$$ and consider some ideal generated by elements of the form $(sx_s - 1)$. Considering the polynomial ring as a free $A$-module, the required statement follows from the previous one, once one shows the ideal is generated as an $A$-submodule of $B$ by elements of the same form as above. This is not so bad since $$(sx_s -1)(tx_t - 1) = (stx_sx_t - 1) - (sx_s -1) - (tx_t - 1)$$ is already a sum of three relations, and the same will occur for more complicated monomials in place of $x_s$ and $x_t$. Multiplying a relation by a monomial also gives you a sum of relations in the quotient submodule of $$bigoplus_{i,j}M_i,j$$ above.






            share|cite|improve this answer











            $endgroup$


















              1












              $begingroup$

              I think the main mistake is in the statement "by degree considerations..." I did not verify that it is true for integral domains, but these considerations are more complicated in the presence of zerodivisors.



              Let's consider a (classical) localization in a ring $A$ with zerodivisors. For example, let $A = mathbb{Z}[y]/(2y)$ and invert $2$. Then in the localization $y=0$.



              Now consider your polynomial-ring construction, in which $2x_2 - 1 = 0$ and (multiplying by $y$) $$2x_2 y - y = 0,$$ i.e. $y=0$. This looks like a separate, more minor, problem: it is a relation not of the form $$frac{a}{s}=frac{b}{t}$$ for $s,t in S$ (you might want to take $1 in S$.) I'll assume we took $1in S$, and use a corresponding variable $x_1$ below.



              Anyway, $y=0$ and in particular we have for example $$yx_2 - 0x_1 = yx_2 in I.$$ So by your degree considerations, we should get $yx_2 = c(2x_2 - 1) - c(1x_1-1)$ in $A[x_1, x_2]$ for some $cin A$. This is not the case. To exhibit this relation as an element of $I$ you need something like $yx_2 cdot (2x_2 - 1),$ where $yx_2$ is not of degree $0$, hence not in $A$, so the degree considerations fail.





              I previously wrote a "correction" to your proof, but it was mainly nonsense. There was some germ of a good idea hidden there, though. Here is a sketch of a correct proof:



              Consider a ring $A$ and a module $M$ over it, and take $$(Moplus M) / ((x,sx))$$ for some fixed $sin A$. What is the submodule generated by the first coordinate? If an element $x$ is annihilated by $s$, then $$(x,0) = (x,0) - (x,sx) = (0,0)$$ in the quotient. It can be verified directly that the submodule is just $$frac{M}{text{ker}(s:Mrightarrow M)}.$$ What if we take $$bigoplus_{i,j ge 0} M$$ and mod out by elements of the form $$se_{i+1,j}-e_{i,j},quad te_{i,j+1}-e_{i,j}?$$ I claim the $e_{0,0}$ coordinate then generates a module isomorphic to $$frac{M}{bigcup_n text{ker}(st)^n:Mrightarrow M},$$ et cetera.



              To see this, consider the colimit explicitly. If an element of the submodule generated by $M_{0,0}$ in the quotient vanishes, it vanishes already in some f.g. submodule, which is contained in some direct sum of a finite set of $M_{i,j}$, and hence in a "rectangular" submodule $$bigoplus_{0le i,jle n} M_{i,j}.$$ Any element here can be "pushed" to one supported only in $M_{n,n}$, and it is easy to check what it means that it vanishes there. The same argument generalizes to higher "dimensions" $$bigoplus M_{i,j,k}.$$



              You want to take $$B:= Aleft[{x_s}_{sin S}right]$$ and consider some ideal generated by elements of the form $(sx_s - 1)$. Considering the polynomial ring as a free $A$-module, the required statement follows from the previous one, once one shows the ideal is generated as an $A$-submodule of $B$ by elements of the same form as above. This is not so bad since $$(sx_s -1)(tx_t - 1) = (stx_sx_t - 1) - (sx_s -1) - (tx_t - 1)$$ is already a sum of three relations, and the same will occur for more complicated monomials in place of $x_s$ and $x_t$. Multiplying a relation by a monomial also gives you a sum of relations in the quotient submodule of $$bigoplus_{i,j}M_i,j$$ above.






              share|cite|improve this answer











              $endgroup$
















                1












                1








                1





                $begingroup$

                I think the main mistake is in the statement "by degree considerations..." I did not verify that it is true for integral domains, but these considerations are more complicated in the presence of zerodivisors.



                Let's consider a (classical) localization in a ring $A$ with zerodivisors. For example, let $A = mathbb{Z}[y]/(2y)$ and invert $2$. Then in the localization $y=0$.



                Now consider your polynomial-ring construction, in which $2x_2 - 1 = 0$ and (multiplying by $y$) $$2x_2 y - y = 0,$$ i.e. $y=0$. This looks like a separate, more minor, problem: it is a relation not of the form $$frac{a}{s}=frac{b}{t}$$ for $s,t in S$ (you might want to take $1 in S$.) I'll assume we took $1in S$, and use a corresponding variable $x_1$ below.



                Anyway, $y=0$ and in particular we have for example $$yx_2 - 0x_1 = yx_2 in I.$$ So by your degree considerations, we should get $yx_2 = c(2x_2 - 1) - c(1x_1-1)$ in $A[x_1, x_2]$ for some $cin A$. This is not the case. To exhibit this relation as an element of $I$ you need something like $yx_2 cdot (2x_2 - 1),$ where $yx_2$ is not of degree $0$, hence not in $A$, so the degree considerations fail.





                I previously wrote a "correction" to your proof, but it was mainly nonsense. There was some germ of a good idea hidden there, though. Here is a sketch of a correct proof:



                Consider a ring $A$ and a module $M$ over it, and take $$(Moplus M) / ((x,sx))$$ for some fixed $sin A$. What is the submodule generated by the first coordinate? If an element $x$ is annihilated by $s$, then $$(x,0) = (x,0) - (x,sx) = (0,0)$$ in the quotient. It can be verified directly that the submodule is just $$frac{M}{text{ker}(s:Mrightarrow M)}.$$ What if we take $$bigoplus_{i,j ge 0} M$$ and mod out by elements of the form $$se_{i+1,j}-e_{i,j},quad te_{i,j+1}-e_{i,j}?$$ I claim the $e_{0,0}$ coordinate then generates a module isomorphic to $$frac{M}{bigcup_n text{ker}(st)^n:Mrightarrow M},$$ et cetera.



                To see this, consider the colimit explicitly. If an element of the submodule generated by $M_{0,0}$ in the quotient vanishes, it vanishes already in some f.g. submodule, which is contained in some direct sum of a finite set of $M_{i,j}$, and hence in a "rectangular" submodule $$bigoplus_{0le i,jle n} M_{i,j}.$$ Any element here can be "pushed" to one supported only in $M_{n,n}$, and it is easy to check what it means that it vanishes there. The same argument generalizes to higher "dimensions" $$bigoplus M_{i,j,k}.$$



                You want to take $$B:= Aleft[{x_s}_{sin S}right]$$ and consider some ideal generated by elements of the form $(sx_s - 1)$. Considering the polynomial ring as a free $A$-module, the required statement follows from the previous one, once one shows the ideal is generated as an $A$-submodule of $B$ by elements of the same form as above. This is not so bad since $$(sx_s -1)(tx_t - 1) = (stx_sx_t - 1) - (sx_s -1) - (tx_t - 1)$$ is already a sum of three relations, and the same will occur for more complicated monomials in place of $x_s$ and $x_t$. Multiplying a relation by a monomial also gives you a sum of relations in the quotient submodule of $$bigoplus_{i,j}M_i,j$$ above.






                share|cite|improve this answer











                $endgroup$



                I think the main mistake is in the statement "by degree considerations..." I did not verify that it is true for integral domains, but these considerations are more complicated in the presence of zerodivisors.



                Let's consider a (classical) localization in a ring $A$ with zerodivisors. For example, let $A = mathbb{Z}[y]/(2y)$ and invert $2$. Then in the localization $y=0$.



                Now consider your polynomial-ring construction, in which $2x_2 - 1 = 0$ and (multiplying by $y$) $$2x_2 y - y = 0,$$ i.e. $y=0$. This looks like a separate, more minor, problem: it is a relation not of the form $$frac{a}{s}=frac{b}{t}$$ for $s,t in S$ (you might want to take $1 in S$.) I'll assume we took $1in S$, and use a corresponding variable $x_1$ below.



                Anyway, $y=0$ and in particular we have for example $$yx_2 - 0x_1 = yx_2 in I.$$ So by your degree considerations, we should get $yx_2 = c(2x_2 - 1) - c(1x_1-1)$ in $A[x_1, x_2]$ for some $cin A$. This is not the case. To exhibit this relation as an element of $I$ you need something like $yx_2 cdot (2x_2 - 1),$ where $yx_2$ is not of degree $0$, hence not in $A$, so the degree considerations fail.





                I previously wrote a "correction" to your proof, but it was mainly nonsense. There was some germ of a good idea hidden there, though. Here is a sketch of a correct proof:



                Consider a ring $A$ and a module $M$ over it, and take $$(Moplus M) / ((x,sx))$$ for some fixed $sin A$. What is the submodule generated by the first coordinate? If an element $x$ is annihilated by $s$, then $$(x,0) = (x,0) - (x,sx) = (0,0)$$ in the quotient. It can be verified directly that the submodule is just $$frac{M}{text{ker}(s:Mrightarrow M)}.$$ What if we take $$bigoplus_{i,j ge 0} M$$ and mod out by elements of the form $$se_{i+1,j}-e_{i,j},quad te_{i,j+1}-e_{i,j}?$$ I claim the $e_{0,0}$ coordinate then generates a module isomorphic to $$frac{M}{bigcup_n text{ker}(st)^n:Mrightarrow M},$$ et cetera.



                To see this, consider the colimit explicitly. If an element of the submodule generated by $M_{0,0}$ in the quotient vanishes, it vanishes already in some f.g. submodule, which is contained in some direct sum of a finite set of $M_{i,j}$, and hence in a "rectangular" submodule $$bigoplus_{0le i,jle n} M_{i,j}.$$ Any element here can be "pushed" to one supported only in $M_{n,n}$, and it is easy to check what it means that it vanishes there. The same argument generalizes to higher "dimensions" $$bigoplus M_{i,j,k}.$$



                You want to take $$B:= Aleft[{x_s}_{sin S}right]$$ and consider some ideal generated by elements of the form $(sx_s - 1)$. Considering the polynomial ring as a free $A$-module, the required statement follows from the previous one, once one shows the ideal is generated as an $A$-submodule of $B$ by elements of the same form as above. This is not so bad since $$(sx_s -1)(tx_t - 1) = (stx_sx_t - 1) - (sx_s -1) - (tx_t - 1)$$ is already a sum of three relations, and the same will occur for more complicated monomials in place of $x_s$ and $x_t$. Multiplying a relation by a monomial also gives you a sum of relations in the quotient submodule of $$bigoplus_{i,j}M_i,j$$ above.







                share|cite|improve this answer














                share|cite|improve this answer



                share|cite|improve this answer








                edited Feb 3 at 13:49

























                answered Feb 1 at 22:40









                gyashfegyashfe

                963




                963






























                    draft saved

                    draft discarded




















































                    Thanks for contributing an answer to Mathematics Stack Exchange!


                    • Please be sure to answer the question. Provide details and share your research!

                    But avoid



                    • Asking for help, clarification, or responding to other answers.

                    • Making statements based on opinion; back them up with references or personal experience.


                    Use MathJax to format equations. MathJax reference.


                    To learn more, see our tips on writing great answers.




                    draft saved


                    draft discarded














                    StackExchange.ready(
                    function () {
                    StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3095680%2fequality-of-fractions-in-localization-as-quotient%23new-answer', 'question_page');
                    }
                    );

                    Post as a guest















                    Required, but never shown





















































                    Required, but never shown














                    Required, but never shown












                    Required, but never shown







                    Required, but never shown

































                    Required, but never shown














                    Required, but never shown












                    Required, but never shown







                    Required, but never shown







                    Popular posts from this blog

                    MongoDB - Not Authorized To Execute Command

                    in spring boot 2.1 many test slices are not allowed anymore due to multiple @BootstrapWith

                    Npm cannot find a required file even through it is in the searched directory