Fractional programming GudangMovies21 Rebahinxxi LK21

      In mathematical optimization, fractional programming is a generalization of linear-fractional programming. The objective function in a fractional program is a ratio of two functions that are in general nonlinear. The ratio to be optimized often describes some kind of efficiency of a system.


      Definition


      Let



      f
      ,
      g
      ,

      h

      j


      ,
      j
      =
      1
      ,

      ,
      m


      {\displaystyle f,g,h_{j},j=1,\ldots ,m}

      be real-valued functions defined on a set





      S


      0





      R


      n




      {\displaystyle \mathbf {S} _{0}\subset \mathbb {R} ^{n}}

      . Let




      S

      =
      {

      x




      S


      0


      :

      h

      j


      (

      x

      )

      0
      ,
      j
      =
      1
      ,

      ,
      m
      }


      {\displaystyle \mathbf {S} =\{{\boldsymbol {x}}\in \mathbf {S} _{0}:h_{j}({\boldsymbol {x}})\leq 0,j=1,\ldots ,m\}}

      . The nonlinear program






      maximize


      x



      S








      f
      (

      x

      )


      g
      (

      x

      )



      ,


      {\displaystyle {\underset {{\boldsymbol {x}}\in \mathbf {S} }{\text{maximize}}}\quad {\frac {f({\boldsymbol {x}})}{g({\boldsymbol {x}})}},}


      where



      g
      (

      x

      )
      >
      0


      {\displaystyle g({\boldsymbol {x}})>0}

      on




      S



      {\displaystyle \mathbf {S} }

      , is called a fractional program.


      Concave fractional programs


      A fractional program in which f is nonnegative and concave, g is positive and convex, and S is a convex set is called a concave fractional program. If g is affine, f does not have to be restricted in sign. The linear fractional program is a special case of a concave fractional program where all functions



      f
      ,
      g
      ,

      h

      j


      ,
      j
      =
      1
      ,

      ,
      m


      {\displaystyle f,g,h_{j},j=1,\ldots ,m}

      are affine.


      = Properties

      =
      The function



      q
      (

      x

      )
      =
      f
      (

      x

      )

      /

      g
      (

      x

      )


      {\displaystyle q({\boldsymbol {x}})=f({\boldsymbol {x}})/g({\boldsymbol {x}})}

      is semistrictly quasiconcave on S. If f and g are differentiable, then q is pseudoconcave. In a linear fractional program, the objective function is pseudolinear.


      = Transformation to a concave program

      =
      By the transformation




      y

      =


      x

      g
      (

      x

      )



      ;
      t
      =


      1

      g
      (

      x

      )





      {\displaystyle {\boldsymbol {y}}={\frac {\boldsymbol {x}}{g({\boldsymbol {x}})}};t={\frac {1}{g({\boldsymbol {x}})}}}

      , any concave fractional program can be transformed to the equivalent parameter-free concave program










      maximize



      y
      t





      S


      0








      t
      f

      (


      y
      t


      )






      subject to




      t
      g

      (


      y
      t


      )


      1
      ,





      t

      0.






      {\displaystyle {\begin{aligned}{\underset {{\frac {\boldsymbol {y}}{t}}\in \mathbf {S} _{0}}{\text{maximize}}}\quad &tf\left({\frac {\boldsymbol {y}}{t}}\right)\\{\text{subject to}}\quad &tg\left({\frac {\boldsymbol {y}}{t}}\right)\leq 1,\\&t\geq 0.\end{aligned}}}


      If g is affine, the first constraint is changed to



      t
      g
      (


      y
      t


      )
      =
      1


      {\displaystyle tg({\frac {\boldsymbol {y}}{t}})=1}

      and the assumption that g is positive may be dropped. Also, it simplifies to



      g
      (

      y

      )
      =
      1


      {\displaystyle g({\boldsymbol {y}})=1}

      .


      = Duality

      =
      The Lagrangian dual of the equivalent concave program is










      minimize
      u







      sup


      x




      S


      0








      f
      (

      x

      )



      u


      T



      h

      (

      x

      )


      g
      (

      x

      )








      subject to





      u

      i



      0
      ,

      i
      =
      1
      ,

      ,
      m
      .






      {\displaystyle {\begin{aligned}{\underset {\boldsymbol {u}}{\text{minimize}}}\quad &{\underset {{\boldsymbol {x}}\in \mathbf {S} _{0}}{\operatorname {sup} }}{\frac {f({\boldsymbol {x}})-{\boldsymbol {u}}^{T}{\boldsymbol {h}}({\boldsymbol {x}})}{g({\boldsymbol {x}})}}\\{\text{subject to}}\quad &u_{i}\geq 0,\quad i=1,\dots ,m.\end{aligned}}}



      Notes




      References


      Avriel, Mordecai; Diewert, Walter E.; Schaible, Siegfried; Zang, Israel (1988). Generalized Concavity. Plenum Press.
      Schaible, Siegfried (1983). "Fractional programming". Zeitschrift für Operations Research. 27: 39–54. doi:10.1007/bf01916898. S2CID 28766871.

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