# Number of functions $f:\{1, 2, \ldots, n\} \to \{0, 1\}$ that assign $1$ to exactly one positive integer less than $n$

**I've seen this question is discrete maths text :

How many functions are there from the set $\{1, 2, . . . , n\},$ where $n$ is a positive integer, to the set $\{0, 1\}.$

a) that assign to $1$ exactly one of the positive integers less than $n$?

The answer for this question is $2(n-1).$

Shouldn't the answer be $2^{n-1}?$

My Idea for that answer : There will be total of $2^n$ functions. And the condition says that it has to assign '1' to integers less than 'n' which means there will be $(n-1)$ possibilities.

So the answer should be $2^{n-1}$ functions ..Right? Explain please.

• How many ways are there to pick the single element less than $n$ that gets the value $1$? Once you've done that, how many ways are there to finish the function? May 3, 2015 at 12:37

The functions to be counted are those that assign to $1$ exactly one (one and only one) of the integers less than $n$ (one of the elements in $\{1, 2, \ldots, n-1\}$. Once that assignment is determined $(n-1$ choice $)$, the remaining elements in $\{1, 2, \ldots, n-1\}$ must then be assigned to $0$.
Then $n$ itself can be assigned to $0$ or $1$.
So there are two choices for assigning $n$, and $n-1$ choices to assign to $1$.
That gives $2(n-1)$ possible functions that satisfy the given requirement.
• Thank you very much Jordan. Can you please explain why we can not say, there are $(n-1)$ choices to assign to 1 and there are $(n-1)$ choices to assign them to 0 and finally 2 for $n$, so we have $(n-1) + (n-1) + 2$.