No, take *y = sqrt[x^(2)]* for example. We define this function to give *y = |x|* because otherwise it wouldn't be bijectiv.

Let's assume we apply a 'function' (a relation to word it better) *y = f(x)* s.t. *y = x^2*. This function isn't bijectiv, as every *y* has two *x* we can assign to it. E.g. *y = 4* is true for *x = 2* and *x = -2*.

Saying: this function is surjectiv but not injectiv, thus not bijectiv.