Hi everybody

Could anyone please tell me how to check or prove if a function from [0,1]^2 to [0,1]^2 is a bijection. It easy from [0,1] to [0,1], but not so obvious in the 2 dimensional case. The type of function I am thinking of is continuous but not necessarly differentiable (e.g I don't want to have to use partial derivatives)

thanks