That's not proper syntax. You're trying to both call and declare a function simultaneously. You can't do that.
Try it this way
H = linspace(0, 1, 1000);
moyenne = M(1-(1 ./ sinh(J ./ 2*kb*T).^4)) .^ (1/8);
plot(H, moyenne)
I'm still not sure about that weird expression. Hopefully J or T or M has 1000 elements. If M is an array rather than some other function, then you need to make sure the expression inside is non-zero, non-negative, and non-fractional numbers. And I don't see how H is involved in any way in your expression for moyenne. So I still think it won't work until you define what things are properly.
