When we say that

$f$ is a function from $A$ to $B$

is this different from saying

$f$ is a function from $A$ into $B$

I know what injective ("1-1"), surjective ("onto"), and bijective functions are, but is there such a thing as an "into" function?

  • 3
    $\begingroup$ some people prefer "onto" instead of "surjective", and then "into" makes sense as the analogue of "injective", but i think most people would just interpret it as the same as "$f$ is a function from $A$ to $B$" $\endgroup$ Jun 14, 2013 at 17:34
  • 9
    $\begingroup$ I think this is a perfectly fair question, unsure why it was downvoted. $\endgroup$
    – icurays1
    Jun 14, 2013 at 17:36
  • 5
    $\begingroup$ As a non-english motherboard I'd kill anyone that uses this kind of terminology, since into, onto, to seem more or less the same and are easily confused. I'd personally say "$f$ is an injection/surjection from $A$ to $B$" if I want to add that information without adding adjectives. $\endgroup$
    – Bakuriu
    Jun 14, 2013 at 21:10

4 Answers 4


Both expressions say the same thing.

But note that saying "$f: A \to B$ is a function from $A$ into $B$ does not imply that $f$ is into but not onto (i.e., it does not rule out that $f$ might be onto, so it is not the "converse" of, or the negation of, the descriptor "onto" or "surjective"). Indeed, it says nothing more, and nothing less, than the alternative: "$f$ is a function from $A$ to $B$."

  • So we can either say that there is no such thing as an "into function" (i.e. it is NOT used to describe some, but not all functions, nor is it describe particular kinds of functions),

  • Or we can say that every function is into (meaning every function $f: A \to B$ is a function on $A$ into $B$.)

  • $\begingroup$ Nice write up! +1 $\endgroup$
    – Amzoti
    Jun 15, 2013 at 1:16
  • $\begingroup$ Thank you for the edit, @ruakh! $\endgroup$
    – amWhy
    Jun 15, 2013 at 2:11
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    $\begingroup$ As Celine songs: Fly, Fly little one....:-) $\endgroup$
    – Mikasa
    Jun 15, 2013 at 5:06
  • $\begingroup$ @amWhy , thanx , one strange thing is that i have 17+ upvotes ! when i asked the question , i hoped that it will not be closed because it doesn't satisfies the politics of the site as it has no useful information for others in future ! $\endgroup$
    – FNH
    Jun 15, 2013 at 13:40
  • 1
    $\begingroup$ Thanks, MathsLover! I love math as much as you ;-) $\endgroup$
    – amWhy
    Jun 16, 2013 at 17:56

Both mean the same thing. There is no such thing as an "into" function.


Any function $f : A \to B$ is said to map $A$ into $B$ or to be a mapping from $A$ into $B$. The term into is used in general for any function, it doesn't relate to any specific kind of functions.


There is no difference between "to" and "into" here.

In Gelbaum and Olmstead's Counterexamples in Analysis, they use this terminology in a way that makes it seem slightly more reasonable (though I personally still wouldn't ever use "into").

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