Question 209702
Function  can be  defined  as  an  expression  that  has  exactly  one output  for  every  input,,  or  passes  the  vertical  line  test.
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for [y] = 4-x,,,a  T  box  might  look  like
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input(x),,,,output(y)
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0,,,,,,,,,,,+/- 4
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2,,,,,,,,,,,+/- 2
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4,,,,,,,,,,,+/- 0
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-4,,,,,,,,,,+/- 8
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We  find  more  than  one  output  possible  for  single  input,,,,therefore  not  a  function.
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A quick  sketch  of  the  above  data, shows  that  it  also  fails  the  vertical  line  test,  therefore not  a  function.
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