# Which function should I choose?

I am writing a paper and looking for a function.Its shape is just like this:

when its x value is very small ,its y is close to 0 , but when x value is a little big, then its y value is very close to 1.

Anyone can tell me ?

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$\tanh x$ (hyperbolic tangent) and $\operatorname{erf} x$ (error function) both have shapes like this. – Antonio Vargas Jul 8 '14 at 3:46
What paper is it about? Why are you looking for such function? – Gina Jul 8 '14 at 4:12