MATLAB: Multi objective optimization of a 2D function.

optimization

I need to optimize the following function,
.
I have written the following program. But, I couldn't figure out how to get the program working with two parameters.
main.m
 fitness = @simpleMultiObjective;
 nvar = 2;
 [x, fval] = gamultiobj(fitness, nvar);
simpleMultiObjective.m
 function z = simpleMultiObjective(x, y)
    z(1) = x.^4 + y.^4 - x.^2 * y.^2 + x * y - 10 * x.^2;    
    z(2) = x.^4 + y.^4 - x.^2 * y.^2 + x * y;
 end
This program is generating the following error,
>> main
Error using simpleMultiObjective (line 2)
Not enough input arguments.
Error in createAnonymousFcn>@(x)fcn(x,FcnArgs{:}) (line 11)
fcn_handle = @(x) fcn(x,FcnArgs{:});
Error in gamultiobjMakeState (line 27)
    Score = FitnessFcn(state.Population(1,:));
Error in gamultiobjsolve (line 8)
state = gamultiobjMakeState(GenomeLength,FitnessFcn,ConstrFcn,output.problemtype,options);
Error in gamultiobj (line 261)
[x,fval,exitFlag,output,population,scores] = gamultiobjsolve(FitnessFcn,nvars, ...
Error in main (line 4)
[x, fval] = gamultiobj(fitness, nvar);
Caused by:
    Failure in initial user-supplied fitness function evaluation. GAMULTIOBJ cannot continue.
>>

Best Answer

 function z = simpleMultiObjective(xy)
    x = xy(1); y = xy(2);
    z(1) = x.^4 + y.^4 - x.^2 * y.^2 + x * y - 10 * x.^2;    
    z(2) = x.^4 + y.^4 - x.^2 * y.^2 + x * y;
 end
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