octave-3.0.3:2> y0 = [0; 0; 10; 10;] % x1, x2, v1, v2
y0 =

    0
    0
   10
   10

octave-3.0.3:3> f = @(y) return [y(1), y(2), 0, -9,81]
parse error:

  syntax error

>>> f = @(y) return [y(1), y(2), 0, -9,81]
                    ^

octave-3.0.3:3> f = @(y) return [y(1); y(2); 0; -9,81]
parse error:

  syntax error

>>> f = @(y) return [y(1); y(2); 0; -9,81]
                    ^

octave-3.0.3:3> f = @(y) [y(1); y(2); 0; -9,81]
f =

@(y) [y(1); y(2); 0; -9, 81]

octave-3.0.3:4> f(y0)
error: number of columns must match (2 != 1)
error: called from `?unknown?'
octave-3.0.3:4> f = @(y) [y(1,:); y(2,:); 0; -9,81]
f =

@(y) [y(1, :); y(2, :); 0; -9, 81]

octave-3.0.3:5> f(y0)
error: number of columns must match (2 != 1)
error: called from `?unknown?'
octave-3.0.3:5> y0(1)
ans = 0
octave-3.0.3:6> y0(2)
ans = 0
octave-3.0.3:7> y0(3)
ans =  10
octave-3.0.3:8> f = @(y) [y(1); y(2); 0; -9,81]
f =

@(y) [y(1); y(2); 0; -9, 81]

octave-3.0.3:9> f(y0)
error: number of columns must match (2 != 1)
error: called from `?unknown?'
octave-3.0.3:9> f = @(y(1)) [y(1); y(2); 0; -9,81]
parse error:

  syntax error

>>> f = @(y(1)) [y(1); y(2); 0; -9,81]
           ^

parse error:

  invalid parameter list

>>> f = @(y(1)) [y(1); y(2); 0; -9,81]
           ^

octave-3.0.3:9> f = @(yy) [yy(1); yy(2); 0; -9,81]
f =

@(yy) [yy(1); yy(2); 0; -9, 81]

octave-3.0.3:10> f(y0)
error: number of columns must match (2 != 1)
error: called from `?unknown?'
octave-3.0.3:10> f([2 3; 4 5])
error: number of columns must match (2 != 1)
error: called from `?unknown?'
octave-3.0.3:10> f = @(y) [ y(1); y(2); 0; -9.81 ]
f =

@(y) [y(1); y(2); 0; -9.81]

octave-3.0.3:11> f(y0)
ans =

   0.00000
   0.00000
   0.00000
  -9.81000

octave-3.0.3:12> f = @(y) [ y(3); y(4); 0; -9.81 ]
f =

@(y) [y(3); y(4); 0; -9.81]

octave-3.0.3:13> f(y0)
ans =

   10.00000
   10.00000
    0.00000
   -9.81000

octave-3.0.3:14>