{
  "index": "1951-B-1",
  "type": "ANA",
  "tag": [
    "ANA",
    "ALG"
  ],
  "difficulty": "",
  "question": "1. Find the condition that the functions \\( M(x, y) \\) and \\( N(x, y) \\) must satisfy in order that the differential equation \\( M d x+N d y=0 \\) shall have an integrating factor of the form \\( f(x y) \\). You may assume that \\( M \\) and \\( N \\) have continuous partial derivatives of all orders.",
  "solution": "Solution. If there is such an integrating factor, say \\( f(x y) \\), then\n\\[\nf(x y) M(x, y) d x+f(x y) N(x, y) d y=0\n\\]\nmust be a closed (i.e., locally exact) differential form. For this it is necessary that\n\\[\n\\frac{\\partial}{\\partial y}[f(x y) M(x, y)]=\\frac{\\partial}{\\partial x}[f(x y) N(x, y)],\n\\]\nthat is,\n\\[\nx f^{\\prime} M+f \\frac{\\partial M}{\\partial y}=y f^{\\prime} N+f \\frac{\\partial N}{\\partial x}\n\\]\nwhence\n\\[\n\\frac{f^{\\prime}}{f}=\\frac{1}{x M-y N}\\left(\\frac{\\partial N}{\\partial x}-\\frac{\\partial M}{\\partial y}\\right)\n\\]\nassuming \\( f(x y) \\neq 0 \\) and \\( x M-y N \\neq 0 \\).\nNow the left-hand side of (4) is a function of ( \\( x y \\) ), hence the right-hand side must be also. Thus, assuming \\( x M-y N \\neq 0 \\), a necessary condition for the existence of the desired integrating factor is that there exist a function \\( R \\) of one variable such that\n\\[\n\\frac{1}{x M-y N}\\left(\\frac{\\partial N}{\\partial x}-\\frac{\\partial M}{\\partial y}\\right)=R(x y) .\n\\]\n\nConversely, if such a function \\( R \\) exists, then an integrating factor is given by\n\\[\nf(x y)=\\exp \\int^{x y} R(t) d t\n\\]\nsince this function will certainly satisfy (2).\nJustification. The theory of integrating factors is usually considered as a local theory; that is, one seeks to convert a differential form into one that is only locally exact and for this the cross derivative condition is both necessary and sufficient. Hence (2) is both necessary and sufficient to find a local integrating factor.\n\nThe condition \\( f \\neq 0 \\) involved in (4) was not investigated. Suppose \\( x M- \\) \\( y N \\) does not vanish at a point \\( \\left(x_{0}, y_{0}\\right) \\). By continuity we confine ourselves to a convex open neighborhood \\( U \\) of \\( \\left(x_{0}, y_{0}\\right) \\) on which \\( x M-y N \\) never vanishes. If \\( f(x y) \\) is an integrating factor defined on \\( U \\) and \\( f\\left(x_{0} y_{0}\\right)=0 \\), then we may regard (3), restricted to some straight line through \\( \\left(x_{0}, y_{0}\\right) \\) as a non-singular homogeneous linear differential equation satisfied by \\( f \\). But the solution of such an equation, if zero at one point, is zero everywhere. Thus if \\( f\\left(x_{0} y_{0}\\right)=0, f \\) is everywhere zero. But, by definition, an integrating factor is not identically zero. Hence the condition stated is indeed necessary and sufficient for the existence of a local integrating factor near a point at which \\( x M-y N \\) does not vanish. The situation near a point at which \\( x M-y N \\) vanishes seems to be complicated.\n\nA differential condition can be found that tells whether or not a smooth function \\( L(x, y) \\) can be expressed in the form \\( R(x y) \\). It is evidently necessary that\n\\[\nx \\frac{\\partial L}{\\partial x}=y \\frac{\\partial L}{\\partial y}\n\\]\nsince if \\( L(x, y)=R(x y) \\), then \\( \\partial L / \\partial x=y R^{\\prime}(x y) \\) and \\( \\partial L / \\partial y=x R^{\\prime}(x y) \\).\nConversely (5) is sufficient locally at any point except the origin. To prove this note that if (5) holds, then\n\\[\n\\frac{d}{d t} L\\left(t, \\frac{a}{t}\\right)=0\n\\]\nfor any fixed \\( a \\), and therefore \\( L \\) is constant along any connected set on which \\( x y \\) is constant. It is clear that this implies that \\( L \\) can be written as \\( R(x y) \\) locally, except at the origin.\n\nOur previous condition becomes\n(6) \\( [x M-y N]\\left[x \\frac{\\partial^{2} N}{\\partial x^{2}}-x \\frac{\\partial^{2} M}{\\partial x \\partial y}-y \\frac{\\partial^{2} N}{\\partial x \\partial y}+y \\frac{\\partial^{2} M}{\\partial y^{2}}\\right] \\)\n\\[\n=\\left[\\frac{\\partial N}{\\partial x}-\\frac{\\partial M}{\\partial y}\\right]\\left[x M+y N+x^{2} \\frac{\\partial M}{\\partial x}+y^{2} \\frac{\\partial N}{\\partial y}-x y\\left(\\frac{\\partial N}{\\partial x}+\\frac{\\partial M}{\\partial y}\\right)\\right]\n\\]\nand we can assert that (6) is a necessary and sufficient condition for the local existence of an integrating factor of the form \\( f(x y) \\) near a point \\( \\left(x_{0}, y_{0}\\right) \\) at which \\( x M-y N \\) does not vanish.\n\nRemark. Condition (5) is not sufficient that \\( L \\) be expressible as \\( R(x y) \\) in the neighborhood of the origin, for we can take \\( L(x, y)=|x| x y^{2} \\), which is a \\( C^{1} \\)-function that satisfies (5) everywhere, but which cannot be written as \\( R(x y) \\) in any neighborhood of the origin.",
  "vars": [
    "x",
    "y",
    "x_0",
    "y_0",
    "t"
  ],
  "params": [
    "M",
    "N",
    "f",
    "R",
    "a",
    "L",
    "U",
    "C"
  ],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "x": "xcoord",
        "y": "ycoord",
        "x_0": "initxco",
        "y_0": "inityco",
        "t": "paramet",
        "M": "funcm",
        "N": "funcn",
        "f": "facfun",
        "R": "singfun",
        "a": "constan",
        "L": "localfn",
        "U": "neighbr",
        "C": "constsc"
      },
      "question": "1. Find the condition that the functions \\( funcm(xcoord, ycoord) \\) and \\( funcn(xcoord, ycoord) \\) must satisfy in order that the differential equation \\( funcm d xcoord+funcn d ycoord=0 \\) shall have an integrating factor of the form \\( facfun(xcoord ycoord) \\). You may assume that \\( funcm \\) and \\( funcn \\) have continuous partial derivatives of all orders.",
      "solution": "Solution. If there is such an integrating factor, say \\( facfun(xcoord ycoord) \\), then\n\\[\nfacfun(xcoord ycoord) funcm(xcoord, ycoord) d xcoord+facfun(xcoord ycoord) funcn(xcoord, ycoord) d ycoord=0\n\\]\nmust be a closed (i.e., locally exact) differential form. For this it is necessary that\n\\[\n\\frac{\\partial}{\\partial ycoord}[facfun(xcoord ycoord) funcm(xcoord, ycoord)]=\\frac{\\partial}{\\partial xcoord}[facfun(xcoord ycoord) funcn(xcoord, ycoord)],\n\\]\nthat is,\n\\[\nxcoord facfun^{\\prime} funcm+facfun \\frac{\\partial funcm}{\\partial ycoord}=ycoord facfun^{\\prime} funcn+facfun \\frac{\\partial funcn}{\\partial xcoord}\n\\]\nwhence\n\\[\n\\frac{facfun^{\\prime}}{facfun}=\\frac{1}{xcoord funcm-ycoord funcn}\\left(\\frac{\\partial funcn}{\\partial xcoord}-\\frac{\\partial funcm}{\\partial ycoord}\\right)\n\\]\nassuming \\( facfun(xcoord ycoord) \\neq 0 \\) and \\( xcoord funcm-ycoord funcn \\neq 0 \\).\nNow the left-hand side of (4) is a function of ( \\( xcoord ycoord \\) ), hence the right-hand side must be also. Thus, assuming \\( xcoord funcm-ycoord funcn \\neq 0 \\), a necessary condition for the existence of the desired integrating factor is that there exist a function \\( singfun \\) of one variable such that\n\\[\n\\frac{1}{xcoord funcm-ycoord funcn}\\left(\\frac{\\partial funcn}{\\partial xcoord}-\\frac{\\partial funcm}{\\partial ycoord}\\right)=singfun(xcoord ycoord) .\n\\]\n\nConversely, if such a function \\( singfun \\) exists, then an integrating factor is given by\n\\[\nfacfun(xcoord ycoord)=\\exp \\int^{xcoord ycoord} singfun(paramet) d paramet\n\\]\nsince this function will certainly satisfy (2).\nJustification. The theory of integrating factors is usually considered as a local theory; that is, one seeks to convert a differential form into one that is only locally exact and for this the cross derivative condition is both necessary and sufficient. Hence (2) is both necessary and sufficient to find a local integrating factor.\n\nThe condition \\( facfun \\neq 0 \\) involved in (4) was not investigated. Suppose \\( xcoord funcm- \\) \\( ycoord funcn \\) does not vanish at a point \\( \\left(initxco, inityco\\right) \\). By continuity we confine ourselves to a convex open neighborhood \\( neighbr \\) of \\( \\left(initxco, inityco\\right) \\) on which \\( xcoord funcm-ycoord funcn \\) never vanishes. If \\( facfun(xcoord ycoord) \\) is an integrating factor defined on \\( neighbr \\) and \\( facfun\\left(initxco inityco\\right)=0 \\), then we may regard (3), restricted to some straight line through \\( \\left(initxco, inityco\\right) \\) as a non-singular homogeneous linear differential equation satisfied by \\( facfun \\). But the solution of such an equation, if zero at one point, is zero everywhere. Thus if \\( facfun\\left(initxco inityco\\right)=0, facfun \\) is everywhere zero. But, by definition, an integrating factor is not identically zero. Hence the condition stated is indeed necessary and sufficient for the existence of a local integrating factor near a point at which \\( xcoord funcm-ycoord funcn \\) does not vanish. The situation near a point at which \\( xcoord funcm-ycoord funcn \\) vanishes seems to be complicated.\n\nA differential condition can be found that tells whether or not a smooth function \\( localfn(xcoord, ycoord) \\) can be expressed in the form \\( singfun(xcoord ycoord) \\). It is evidently necessary that\n\\[\nxcoord \\frac{\\partial localfn}{\\partial xcoord}=ycoord \\frac{\\partial localfn}{\\partial ycoord}\n\\]\nsince if \\( localfn(xcoord, ycoord)=singfun(xcoord ycoord) \\), then \\( \\partial localfn / \\partial xcoord=ycoord singfun^{\\prime}(xcoord ycoord) \\) and \\( \\partial localfn / \\partial ycoord=xcoord singfun^{\\prime}(xcoord ycoord) \\).\nConversely (5) is sufficient locally at any point except the origin. To prove this note that if (5) holds, then\n\\[\n\\frac{d}{d paramet} localfn\\left(paramet, \\frac{constan}{paramet}\\right)=0\n\\]\nfor any fixed \\( constan \\), and therefore \\( localfn \\) is constant along any connected set on which \\( xcoord ycoord \\) is constant. It is clear that this implies that \\( localfn \\) can be written as \\( singfun(xcoord ycoord) \\) locally, except at the origin.\n\nOur previous condition becomes\n(6) \\( [xcoord funcm-ycoord funcn]\\left[xcoord \\frac{\\partial^{2} funcn}{\\partial xcoord^{2}}-xcoord \\frac{\\partial^{2} funcm}{\\partial xcoord \\partial ycoord}-ycoord \\frac{\\partial^{2} funcn}{\\partial xcoord \\partial ycoord}+ycoord \\frac{\\partial^{2} funcm}{\\partial ycoord^{2}}\\right] \\)\n\\[\n=\\left[\\frac{\\partial funcn}{\\partial xcoord}-\\frac{\\partial funcm}{\\partial ycoord}\\right]\\left[xcoord funcm+ycoord funcn+xcoord^{2} \\frac{\\partial funcm}{\\partial xcoord}+ycoord^{2} \\frac{\\partial funcn}{\\partial ycoord}-xcoord ycoord\\left(\\frac{\\partial funcn}{\\partial xcoord}+\\frac{\\partial funcm}{\\partial ycoord}\\right)\\right]\n\\]\nand we can assert that (6) is a necessary and sufficient condition for the local existence of an integrating factor of the form \\( facfun(xcoord ycoord) \\) near a point \\( \\left(initxco, inityco\\right) \\) at which \\( xcoord funcm-ycoord funcn \\) does not vanish.\n\nRemark. Condition (5) is not sufficient that \\( localfn \\) be expressible as \\( singfun(xcoord ycoord) \\) in the neighborhood of the origin, for we can take \\( localfn(xcoord, ycoord)=|xcoord| xcoord ycoord^{2} \\), which is a \\( constsc^{1} \\)-function that satisfies (5) everywhere, but which cannot be written as \\( singfun(xcoord ycoord) \\) in any neighborhood of the origin."
    },
    "descriptive_long_confusing": {
      "map": {
        "x": "porchlight",
        "y": "drumhandle",
        "x_0": "coppermine",
        "y_0": "lighthouse",
        "t": "stormwater",
        "M": "harborbell",
        "N": "anchorpost",
        "f": "meadowlark",
        "R": "caterpillar",
        "a": "journeyman",
        "L": "crosswinds",
        "U": "riverbank",
        "C": "firewalrus"
      },
      "question": "1. Find the condition that the functions \\( harborbell(porchlight, drumhandle) \\) and \\( anchorpost(porchlight, drumhandle) \\) must satisfy in order that the differential equation \\( harborbell d porchlight+anchorpost d drumhandle=0 \\) shall have an integrating factor of the form \\( meadowlark(porchlight drumhandle) \\). You may assume that \\( harborbell \\) and \\( anchorpost \\) have continuous partial derivatives of all orders.",
      "solution": "Solution. If there is such an integrating factor, say \\( meadowlark(porchlight drumhandle) \\), then\n\\[\nmeadowlark(porchlight drumhandle) harborbell(porchlight, drumhandle) d porchlight+meadowlark(porchlight drumhandle) anchorpost(porchlight, drumhandle) d drumhandle=0\n\\]\nmust be a closed (i.e., locally exact) differential form. For this it is necessary that\n\\[\n\\frac{\\partial}{\\partial drumhandle}[meadowlark(porchlight drumhandle) harborbell(porchlight, drumhandle)]=\\frac{\\partial}{\\partial porchlight}[meadowlark(porchlight drumhandle) anchorpost(porchlight, drumhandle)],\n\\]\nthat is,\n\\[\nporchlight meadowlark^{\\prime} harborbell+meadowlark \\frac{\\partial harborbell}{\\partial drumhandle}=drumhandle meadowlark^{\\prime} anchorpost+meadowlark \\frac{\\partial anchorpost}{\\partial porchlight}\n\\]\nwhence\n\\[\n\\frac{meadowlark^{\\prime}}{meadowlark}=\\frac{1}{porchlight harborbell-drumhandle anchorpost}\\left(\\frac{\\partial anchorpost}{\\partial porchlight}-\\frac{\\partial harborbell}{\\partial drumhandle}\\right)\n\\]\nassuming \\( meadowlark(porchlight drumhandle) \\neq 0 \\) and \\( porchlight harborbell-drumhandle anchorpost \\neq 0 \\).\nNow the left-hand side of (4) is a function of ( \\( porchlight drumhandle \\) ), hence the right-hand side must be also. Thus, assuming \\( porchlight harborbell-drumhandle anchorpost \\neq 0 \\), a necessary condition for the existence of the desired integrating factor is that there exist a function \\( caterpillar \\) of one variable such that\n\\[\n\\frac{1}{porchlight harborbell-drumhandle anchorpost}\\left(\\frac{\\partial anchorpost}{\\partial porchlight}-\\frac{\\partial harborbell}{\\partial drumhandle}\\right)=caterpillar(porchlight drumhandle) .\n\\]\n\nConversely, if such a function \\( caterpillar \\) exists, then an integrating factor is given by\n\\[\nmeadowlark(porchlight drumhandle)=\\exp \\int^{porchlight drumhandle} caterpillar(stormwater) d stormwater\n\\]\nsince this function will certainly satisfy (2).\nJustification. The theory of integrating factors is usually considered as a local theory; that is, one seeks to convert a differential form into one that is only locally exact and for this the cross derivative condition is both necessary and sufficient. Hence (2) is both necessary and sufficient to find a local integrating factor.\n\nThe condition \\( meadowlark \\neq 0 \\) involved in (4) was not investigated. Suppose \\( porchlight harborbell- \\) \\( drumhandle anchorpost \\) does not vanish at a point \\( \\left(coppermine, lighthouse\\right) \\). By continuity we confine ourselves to a convex open neighborhood \\( riverbank \\) of \\( \\left(coppermine, lighthouse\\right) \\) on which \\( porchlight harborbell-drumhandle anchorpost \\) never vanishes. If \\( meadowlark(porchlight drumhandle) \\) is an integrating factor defined on \\( riverbank \\) and \\( meadowlark\\left(coppermine lighthouse\\right)=0 \\), then we may regard (3), restricted to some straight line through \\( \\left(coppermine, lighthouse\\right) \\) as a non-singular homogeneous linear differential equation satisfied by \\( meadowlark \\). But the solution of such an equation, if zero at one point, is zero everywhere. Thus if \\( meadowlark\\left(coppermine lighthouse\\right)=0, meadowlark \\) is everywhere zero. But, by definition, an integrating factor is not identically zero. Hence the condition stated is indeed necessary and sufficient for the existence of a local integrating factor near a point at which \\( porchlight harborbell-drumhandle anchorpost \\) does not vanish. The situation near a point at which \\( porchlight harborbell-drumhandle anchorpost \\) vanishes seems to be complicated.\n\nA differential condition can be found that tells whether or not a smooth function \\( crosswinds(porchlight, drumhandle) \\) can be expressed in the form \\( caterpillar(porchlight drumhandle) \\). It is evidently necessary that\n\\[\nporchlight \\frac{\\partial crosswinds}{\\partial porchlight}=drumhandle \\frac{\\partial crosswinds}{\\partial drumhandle}\n\\]\nsince if \\( crosswinds(porchlight, drumhandle)=caterpillar(porchlight drumhandle) \\), then \\( \\partial crosswinds / \\partial porchlight=drumhandle caterpillar^{\\prime}(porchlight drumhandle) \\) and \\( \\partial crosswinds / \\partial drumhandle=porchlight caterpillar^{\\prime}(porchlight drumhandle) \\).\nConversely (5) is sufficient locally at any point except the origin. To prove this note that if (5) holds, then\n\\[\n\\frac{d}{d stormwater} crosswinds\\left(stormwater, \\frac{journeyman}{stormwater}\\right)=0\n\\]\nfor any fixed \\( journeyman \\), and therefore \\( crosswinds \\) is constant along any connected set on which \\( porchlight drumhandle \\) is constant. It is clear that this implies that \\( crosswinds \\) can be written as \\( caterpillar(porchlight drumhandle) \\) locally, except at the origin.\n\nOur previous condition becomes\n(6) \\( [porchlight harborbell-drumhandle anchorpost]\\left[porchlight \\frac{\\partial^{2} anchorpost}{\\partial porchlight^{2}}-porchlight \\frac{\\partial^{2} harborbell}{\\partial porchlight \\partial drumhandle}-drumhandle \\frac{\\partial^{2} anchorpost}{\\partial porchlight \\partial drumhandle}+drumhandle \\frac{\\partial^{2} harborbell}{\\partial drumhandle^{2}}\\right] \\)\n\\[\n=\\left[\\frac{\\partial anchorpost}{\\partial porchlight}-\\frac{\\partial harborbell}{\\partial drumhandle}\\right]\\left[porchlight harborbell+drumhandle anchorpost+porchlight^{2} \\frac{\\partial harborbell}{\\partial porchlight}+drumhandle^{2} \\frac{\\partial anchorpost}{\\partial drumhandle}-porchlight drumhandle\\left(\\frac{\\partial anchorpost}{\\partial porchlight}+\\frac{\\partial harborbell}{\\partial drumhandle}\\right)\\right]\n\\]\nand we can assert that (6) is a necessary and sufficient condition for the local existence of an integrating factor of the form \\( meadowlark(porchlight drumhandle) \\) near a point \\( \\left(coppermine, lighthouse\\right) \\) at which \\( porchlight harborbell-drumhandle anchorpost \\) does not vanish.\n\nRemark. Condition (5) is not sufficient that \\( crosswinds \\) be expressible as \\( caterpillar(porchlight drumhandle) \\) in the neighborhood of the origin, for we can take \\( crosswinds(porchlight, drumhandle)=|porchlight| porchlight drumhandle^{2} \\), which is a \\( firewalrus^{1} \\)-function that satisfies (5) everywhere, but which cannot be written as \\( caterpillar(porchlight drumhandle) \\) in any neighborhood of the origin."
    },
    "descriptive_long_misleading": {
      "map": {
        "x": "stationary",
        "y": "constantine",
        "x_0": "stationaryzero",
        "y_0": "constantzero",
        "t": "timeless",
        "M": "minimumval",
        "N": "maximumval",
        "f": "fragmenter",
        "R": "oppositefun",
        "a": "variable",
        "L": "nonlinear",
        "U": "closedset",
        "C": "changing"
      },
      "question": "1. Find the condition that the functions \\( minimumval(stationary, constantine) \\) and \\( maximumval(stationary, constantine) \\) must satisfy in order that the differential equation \\( minimumval d stationary+maximumval d constantine=0 \\) shall have an integrating factor of the form \\( fragmenter(stationary constantine) \\). You may assume that \\( minimumval \\) and \\( maximumval \\) have continuous partial derivatives of all orders.",
      "solution": "Solution. If there is such an integrating factor, say \\( fragmenter(stationary constantine) \\), then\n\\[\nfragmenter(stationary constantine) minimumval(stationary, constantine) d stationary+fragmenter(stationary constantine) maximumval(stationary, constantine) d constantine=0\n\\]\nmust be a closed (i.e., locally exact) differential form. For this it is necessary that\n\\[\n\\frac{\\partial}{\\partial constantine}[fragmenter(stationary constantine) minimumval(stationary, constantine)]=\\frac{\\partial}{\\partial stationary}[fragmenter(stationary constantine) maximumval(stationary, constantine)],\n\\]\nthat is,\n\\[\nstationary fragmenter^{\\prime} minimumval+fragmenter \\frac{\\partial minimumval}{\\partial constantine}=constantine fragmenter^{\\prime} maximumval+fragmenter \\frac{\\partial maximumval}{\\partial stationary}\n\\]\nwhence\n\\[\n\\frac{fragmenter^{\\prime}}{fragmenter}=\\frac{1}{stationary minimumval-constantine maximumval}\\left(\\frac{\\partial maximumval}{\\partial stationary}-\\frac{\\partial minimumval}{\\partial constantine}\\right)\n\\]\nassuming \\( fragmenter(stationary constantine) \\neq 0 \\) and \\( stationary minimumval-constantine maximumval \\neq 0 \\).\nNow the left-hand side of (4) is a function of ( \\( stationary constantine \\) ), hence the right-hand side must be also. Thus, assuming \\( stationary minimumval-constantine maximumval \\neq 0 \\), a necessary condition for the existence of the desired integrating factor is that there exist a function \\( oppositefun \\) of one variable such that\n\\[\n\\frac{1}{stationary minimumval-constantine maximumval}\\left(\\frac{\\partial maximumval}{\\partial stationary}-\\frac{\\partial minimumval}{\\partial constantine}\\right)=oppositefun(stationary constantine) .\n\\]\n\nConversely, if such a function \\( oppositefun \\) exists, then an integrating factor is given by\n\\[\nfragmenter(stationary constantine)=\\exp \\int^{stationary constantine} oppositefun(timeless) d timeless\n\\]\nsince this function will certainly satisfy (2).\nJustification. The theory of integrating factors is usually considered as a local theory; that is, one seeks to convert a differential form into one that is only locally exact and for this the cross derivative condition is both necessary and sufficient. Hence (2) is both necessary and sufficient to find a local integrating factor.\n\nThe condition \\( fragmenter \\neq 0 \\) involved in (4) was not investigated. Suppose \\( stationary minimumval-constantine maximumval \\) does not vanish at a point \\( \\left(stationaryzero, constantzero\\right) \\). By continuity we confine ourselves to a convex open neighborhood \\( closedset \\) of \\( \\left(stationaryzero, constantzero\\right) \\) on which \\( stationary minimumval-constantine maximumval \\) never vanishes. If \\( fragmenter(stationary constantine) \\) is an integrating factor defined on \\( closedset \\) and \\( fragmenter\\left(stationaryzero constantzero\\right)=0 \\), then we may regard (3), restricted to some straight line through \\( \\left(stationaryzero, constantzero\\right) \\) as a non-singular homogeneous linear differential equation satisfied by \\( fragmenter \\). But the solution of such an equation, if zero at one point, is zero everywhere. Thus if \\( fragmenter\\left(stationaryzero constantzero\\right)=0, fragmenter \\) is everywhere zero. But, by definition, an integrating factor is not identically zero. Hence the condition stated is indeed necessary and sufficient for the existence of a local integrating factor near a point at which \\( stationary minimumval-constantine maximumval \\) does not vanish. The situation near a point at which \\( stationary minimumval-constantine maximumval \\) vanishes seems to be complicated.\n\nA differential condition can be found that tells whether or not a smooth function \\( nonlinear(stationary, constantine) \\) can be expressed in the form \\( oppositefun(stationary constantine) \\). It is evidently necessary that\n\\[\nstationary \\frac{\\partial nonlinear}{\\partial stationary}=constantine \\frac{\\partial nonlinear}{\\partial constantine}\n\\]\nsince if \\( nonlinear(stationary, constantine)=oppositefun(stationary constantine) \\), then \\( \\partial nonlinear / \\partial stationary=constantine oppositefun^{\\prime}(stationary constantine) \\) and \\( \\partial nonlinear / \\partial constantine=stationary oppositefun^{\\prime}(stationary constantine) \\).\nConversely (5) is sufficient locally at any point except the origin. To prove this note that if (5) holds, then\n\\[\n\\frac{d}{d timeless} nonlinear\\left(timeless, \\frac{variable}{timeless}\\right)=0\n\\]\nfor any fixed \\( variable \\), and therefore \\( nonlinear \\) is constant along any connected set on which \\( stationary constantine \\) is constant. It is clear that this implies that \\( nonlinear \\) can be written as \\( oppositefun(stationary constantine) \\) locally, except at the origin.\n\nOur previous condition becomes\n(6) \\( [stationary minimumval-constantine maximumval]\\left[stationary \\frac{\\partial^{2} maximumval}{\\partial stationary^{2}}-stationary \\frac{\\partial^{2} minimumval}{\\partial stationary \\partial constantine}-constantine \\frac{\\partial^{2} maximumval}{\\partial stationary \\partial constantine}+constantine \\frac{\\partial^{2} minimumval}{\\partial constantine^{2}}\\right] \\)\n\\[\n=\\left[\\frac{\\partial maximumval}{\\partial stationary}-\\frac{\\partial minimumval}{\\partial constantine}\\right]\\left[stationary minimumval+constantine maximumval+stationary^{2} \\frac{\\partial minimumval}{\\partial stationary}+constantine^{2} \\frac{\\partial maximumval}{\\partial constantine}-stationary constantine\\left(\\frac{\\partial maximumval}{\\partial stationary}+\\frac{\\partial minimumval}{\\partial constantine}\\right)\\right]\n\\]\nand we can assert that (6) is a necessary and sufficient condition for the local existence of an integrating factor of the form \\( fragmenter(stationary constantine) \\) near a point \\( \\left(stationaryzero, constantzero\\right) \\) at which \\( stationary minimumval-constantine maximumval \\) does not vanish.\n\nRemark. Condition (5) is not sufficient that \\( nonlinear \\) be expressible as \\( oppositefun(stationary constantine) \\) in the neighborhood of the origin, for we can take \\( nonlinear(stationary, constantine)=|stationary| stationary constantine^{2} \\), which is a \\( changing^{1} \\)-function that satisfies (5) everywhere, but which cannot be written as \\( oppositefun(stationary constantine) \\) in any neighborhood of the origin."
    },
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        "C": "srklypom"
      },
      "question": "1. Find the condition that the functions \\( rvjbfeyd(qzxwvtnp, hjgrksla) \\) and \\( gwhzlskd(qzxwvtnp, hjgrksla) \\) must satisfy in order that the differential equation \\( rvjbfeyd\\, d qzxwvtnp+gwhzlskd\\, d hjgrksla=0 \\) shall have an integrating factor of the form \\( pxtmcrle(qzxwvtnp hjgrksla) \\). You may assume that \\( rvjbfeyd \\) and \\( gwhzlskd \\) have continuous partial derivatives of all orders.",
      "solution": "Solution. If there is such an integrating factor, say \\( pxtmcrle(qzxwvtnp hjgrksla) \\), then\n\\[\npxtmcrle(qzxwvtnp hjgrksla) rvjbfeyd(qzxwvtnp, hjgrksla) d qzxwvtnp+pxtmcrle(qzxwvtnp hjgrksla) gwhzlskd(qzxwvtnp, hjgrksla) d hjgrksla=0\n\\]\nmust be a closed (i.e., locally exact) differential form. For this it is necessary that\n\\[\n\\frac{\\partial}{\\partial hjgrksla}[pxtmcrle(qzxwvtnp hjgrksla) rvjbfeyd(qzxwvtnp, hjgrksla)]=\\frac{\\partial}{\\partial qzxwvtnp}[pxtmcrle(qzxwvtnp hjgrksla) gwhzlskd(qzxwvtnp, hjgrksla)],\n\\]\nthat is,\n\\[\nqzxwvtnp\\, pxtmcrle^{\\prime}\\, rvjbfeyd+pxtmcrle \\frac{\\partial rvjbfeyd}{\\partial hjgrksla}=hjgrksla\\, pxtmcrle^{\\prime}\\, gwhzlskd+pxtmcrle \\frac{\\partial gwhzlskd}{\\partial qzxwvtnp}\n\\]\nwhence\n\\[\n\\frac{pxtmcrle^{\\prime}}{pxtmcrle}=\\frac{1}{qzxwvtnp rvjbfeyd-hjgrksla gwhzlskd}\\left(\\frac{\\partial gwhzlskd}{\\partial qzxwvtnp}-\\frac{\\partial rvjbfeyd}{\\partial hjgrksla}\\right)\n\\]\nassuming \\( pxtmcrle(qzxwvtnp hjgrksla) \\neq 0 \\) and \\( qzxwvtnp rvjbfeyd-hjgrksla gwhzlskd \\neq 0 \\).\nNow the left-hand side of (4) is a function of \\( qzxwvtnp hjgrksla \\), hence the right-hand side must be also. Thus, assuming \\( qzxwvtnp rvjbfeyd-hjgrksla gwhzlskd \\neq 0 \\), a necessary condition for the existence of the desired integrating factor is that there exist a function \\( hbsvqzno \\) of one variable such that\n\\[\n\\frac{1}{qzxwvtnp rvjbfeyd-hjgrksla gwhzlskd}\\left(\\frac{\\partial gwhzlskd}{\\partial qzxwvtnp}-\\frac{\\partial rvjbfeyd}{\\partial hjgrksla}\\right)=hbsvqzno(qzxwvtnp hjgrksla).\n\\]\n\nConversely, if such a function \\( hbsvqzno \\) exists, then an integrating factor is given by\n\\[\npxtmcrle(qzxwvtnp hjgrksla)=\\exp \\int^{qzxwvtnp hjgrksla} hbsvqzno(kmvwqzsb) d kmvwqzsb\n\\]\nsince this function will certainly satisfy (2).\nJustification. The theory of integrating factors is usually considered as a local theory; that is, one seeks to convert a differential form into one that is only locally exact and for this the cross derivative condition is both necessary and sufficient. Hence (2) is both necessary and sufficient to find a local integrating factor.\n\nThe condition \\( pxtmcrle \\neq 0 \\) involved in (4) was not investigated. Suppose \\( qzxwvtnp rvjbfeyd-hjgrksla gwhzlskd \\) does not vanish at a point \\( (bfmdjcqr, znrgtlke) \\). By continuity we confine ourselves to a convex open neighborhood \\( nlxgdewc \\) of \\( (bfmdjcqr, znrgtlke) \\) on which \\( qzxwvtnp rvjbfeyd-hjgrksla gwhzlskd \\) never vanishes. If \\( pxtmcrle(qzxwvtnp hjgrksla) \\) is an integrating factor defined on \\( nlxgdewc \\) and \\( pxtmcrle(bfmdjcqr znrgtlke)=0 \\), then we may regard (3), restricted to some straight line through \\( (bfmdjcqr, znrgtlke) \\) as a non-singular homogeneous linear differential equation satisfied by \\( pxtmcrle \\). But the solution of such an equation, if zero at one point, is zero everywhere. Thus if \\( pxtmcrle(bfmdjcqr znrgtlke)=0, pxtmcrle \\) is everywhere zero. But, by definition, an integrating factor is not identically zero. Hence the condition stated is indeed necessary and sufficient for the existence of a local integrating factor near a point at which \\( qzxwvtnp rvjbfeyd-hjgrksla gwhzlskd \\) does not vanish. The situation near a point at which \\( qzxwvtnp rvjbfeyd-hjgrksla gwhzlskd \\) vanishes seems to be complicated.\n\nA differential condition can be found that tells whether or not a smooth function \\( jkprvafs(qzxwvtnp, hjgrksla) \\) can be expressed in the form \\( hbsvqzno(qzxwvtnp hjgrksla) \\). It is evidently necessary that\n\\[\nqzxwvtnp \\frac{\\partial jkprvafs}{\\partial qzxwvtnp}=hjgrksla \\frac{\\partial jkprvafs}{\\partial hjgrksla}\n\\]\nsince if \\( jkprvafs(qzxwvtnp, hjgrksla)=hbsvqzno(qzxwvtnp hjgrksla) \\), then \\( \\partial jkprvafs / \\partial qzxwvtnp=hjgrksla hbsvqzno^{\\prime}(qzxwvtnp hjgrksla) \\) and \\( \\partial jkprvafs / \\partial hjgrksla=qzxwvtnp hbsvqzno^{\\prime}(qzxwvtnp hjgrksla) \\).\nConversely (5) is sufficient locally at any point except the origin. To prove this note that if (5) holds, then\n\\[\n\\frac{d}{d kmvwqzsb} jkprvafs\\left(kmvwqzsb, \\frac{dxowtreb}{kmvwqzsb}\\right)=0\n\\]\nfor any fixed \\( dxowtreb \\), and therefore \\( jkprvafs \\) is constant along any connected set on which \\( qzxwvtnp hjgrksla \\) is constant. It is clear that this implies that \\( jkprvafs \\) can be written as \\( hbsvqzno(qzxwvtnp hjgrksla) \\) locally, except at the origin.\n\nOur previous condition becomes\n(6) \\( [qzxwvtnp rvjbfeyd-hjgrksla gwhzlskd]\\left[qzxwvtnp \\frac{\\partial^{2} gwhzlskd}{\\partial qzxwvtnp^{2}}-qzxwvtnp \\frac{\\partial^{2} rvjbfeyd}{\\partial qzxwvtnp \\partial hjgrksla}-hjgrksla \\frac{\\partial^{2} gwhzlskd}{\\partial qzxwvtnp \\partial hjgrksla}+hjgrksla \\frac{\\partial^{2} rvjbfeyd}{\\partial hjgrksla^{2}}\\right]\\)\n\\[\n=\\left[\\frac{\\partial gwhzlskd}{\\partial qzxwvtnp}-\\frac{\\partial rvjbfeyd}{\\partial hjgrksla}\\right]\\left[qzxwvtnp rvjbfeyd+hjgrksla gwhzlskd+qzxwvtnp^{2} \\frac{\\partial rvjbfeyd}{\\partial qzxwvtnp}+hjgrksla^{2} \\frac{\\partial gwhzlskd}{\\partial hjgrksla}-qzxwvtnp hjgrksla\\left(\\frac{\\partial gwhzlskd}{\\partial qzxwvtnp}+\\frac{\\partial rvjbfeyd}{\\partial hjgrksla}\\right)\\right]\n\\]\nand we can assert that (6) is a necessary and sufficient condition for the local existence of an integrating factor of the form \\( pxtmcrle(qzxwvtnp hjgrksla) \\) near a point \\( (bfmdjcqr, znrgtlke) \\) at which \\( qzxwvtnp rvjbfeyd-hjgrksla gwhzlskd \\) does not vanish.\n\nRemark. Condition (5) is not sufficient that \\( jkprvafs \\) be expressible as \\( hbsvqzno(qzxwvtnp hjgrksla) \\) in the neighborhood of the origin, for we can take \\( jkprvafs(qzxwvtnp, hjgrksla)=|qzxwvtnp| qzxwvtnp hjgrksla^{2} \\), which is a \\( C^{1} \\)-function that satisfies (5) everywhere, but which cannot be written as \\( hbsvqzno(qzxwvtnp hjgrksla) \\) in any neighborhood of the origin."
    },
    "kernel_variant": {
      "question": "Let 0 < a < b and set  \nD := {(x,y) ∈ ℝ² | a < x² + y² < b} ,\n\ni.e. D is the open annulus whose inner and outer radii are √a and √b.  \nAssume M,N : D → ℝ are C¹–functions.\n\nFor which couples (M,N) does there exist, in a neighbourhood of every point of D, a non–vanishing C¹ function depending only on the radial variable r² = x² + y²\n\n              μ = μ(r²) ≠ 0\n\nsuch that the 1–form\n\n              μ(r²) M(x,y) dx + μ(r²) N(x,y) dy\n\nis exact on that neighbourhood?\n\nGive a necessary and sufficient condition that involves M and N only (no reference to μ).  In formulating the condition you may restrict to those points for which 2y M − 2x N ≠ 0.",
      "solution": "Step 0.  Notation.  Put\n               r² = x² + y² ,       r = √(r²).\nAll derivatives of μ will be taken with respect to its scalar argument r².\n\n────────────────────────────────────────────────────────\n1.  A necessary condition.\n────────────────────────────────────────────────────────\nSuppose that for every p ∈ D there is a neighbourhood U ⊂ D of p and a non–vanishing μ = μ(r²) such that\n               ω := μ(r²)M dx + μ(r²)N dy                     (1)\nis exact on U.  Exactness implies ∂ω/∂y = ∂ω/∂x, that is\n               ∂[μ(r²)M]/∂y = ∂[μ(r²)N]/∂x.                 (2)\nBecause μ depends on x and y only through r², the chain rule gives\n               ∂μ/∂y = 2y μ′,   ∂μ/∂x = 2x μ′.              (3)\nInsert (3) in (2):\n         2y μ′ M + μ M_y = 2x μ′ N + μ N_x ,                (4)\nwhere subscripts denote partial differentiation.  Divide (4) by the non–zero factor μ(r²):\n         (2y M − 2x N) · ( μ′ / μ ) = N_x − M_y.             (5)\nThe quotient μ′ / μ depends only on r²; consequently the right–hand side of (5) must also depend only on r².  Hence we obtain the necessary condition (valid wherever 2yM−2xN ≠ 0)\n\n   (N_x − M_y)/(2yM − 2xN) = R(r²) for some C⁰ function R on (a,b).   (6)\n\n────────────────────────────────────────────────────────\n2.  Sufficiency (local version).\n────────────────────────────────────────────────────────\nConversely, assume condition (6) holds in a neighbourhood U of a point p ∈ D with 2yM − 2xN ≠ 0.  Define\n              μ(r²) := exp( ∫_{r₀²}^{r²} R(t) dt ),          (7)\nwith an arbitrary reference value r₀ satisfying a < r₀² < b.  Then μ is C¹, nowhere zero, and obeys μ′/μ = R.  Substitute this identity into (5); the equality is satisfied identically, so the 1–form ω defined by (1) obeys (2), i.e. is closed.\n\nBecause any sufficiently small Euclidean ball that stays inside the annulus D is simply connected, the classical Poincaré lemma applies: every closed 1–form on such a ball is exact there.  Thus ω is exact on a neighbourhood of p.  Hence condition (6) is sufficient for the local existence of an integrating factor of the desired radial type.\n\n────────────────────────────────────────────────────────\n3.  Points where 2yM − 2xN ≡ 0.\n────────────────────────────────────────────────────────\nIf 2yM − 2xN vanishes identically on some connected component U of D, then equation (5) forces N_x − M_y ≡ 0 on U; the original form M dx + N dy is already closed on U, so taking μ ≡ 1 furnishes the required integrating factor.\n\n────────────────────────────────────────────────────────\n4.  Conclusion.\n────────────────────────────────────────────────────────\nA C¹ integrating factor that depends only on r² exists locally on D if and only if, at every point where 2yM − 2xN ≠ 0, the quotient\n            (N_x − M_y) / (2yM − 2xN)                       (8)\ncan be expressed as a function R of the single variable r² = x² + y².  When that holds one may take μ given by (7); together with the trivial choice μ ≡ 1 on the degenerate sets mentioned in §3 this produces the desired integrating factor throughout D.\n\nRemark on global existence.  The annulus D is not simply connected, so a closed 1–form need not be globally exact.  Therefore the criterion above guarantees only local exactness.  In order that the same μ render the form exact on the whole annulus one has to add the vanishing of its period along any positively oriented circle r = const, i.e.\n            ∮_{|z| = r} μ(r²)\bigl(M dx + N dy\\bigr) = 0   (∀ r with √a < r < √b).\nThis additional global condition is independent of (6) and must be imposed when a single integrating factor on all of D is required.",
      "_meta": {
        "core_steps": [
          "Impose exactness:  ∂(fM)/∂y = ∂(fN)/∂x for a factor f(xy).",
          "Apply the chain rule ⇒ (xM − yN)·f'/f = ∂N/∂x − ∂M/∂y.",
          "Note f'/f depends only on xy, so the right-hand side must equal some R(xy).",
          "Necessity: existence of R(xy) such that (∂N/∂x − ∂M/∂y)/(xM − yN) = R(xy).",
          "Sufficiency: set f(xy)=exp(∫ R(t)dt); this f satisfies the exactness test."
        ],
        "mutable_slots": {
          "slot1": {
            "description": "Specific single argument used for the integrating factor; only its being a smooth function of two variables that collapses to one scalar matters.",
            "original": "xy"
          },
          "slot2": {
            "description": "Degree of smoothness required for M and N; any condition ensuring the derivatives appearing in the chain rule exist and are continuous is enough.",
            "original": "M and N have continuous partial derivatives of all orders"
          }
        }
      }
    }
  },
  "checked": true,
  "problem_type": "proof",
  "iteratively_fixed": true
}