Is there a unique polynomial function $f(x)$ of degree $\lt n$ such that $f(n) = a_n$ where $\{a_n\}$ is a sequence? Is it true that for every sequence $a$ of $n$ numbers there is exactly one polynomial function $f(x)$ of degree $\leq n$ such that all $f(1)=a_1,f(2)=a_2,\dots f(n)=a_{n}$? If so, is there an algorithm to, given the sequence, generate the coefficients of this function?
Intuitively, I feel like this is true, because:


*

*Given $a$ and $b$, you can find a polynomial function $f$ that has degree $1$ such that $f(0)=a$ and $f(1)=b$.

*By induction: given coefficients $a_1,a_2\dots a_n$, you can find a polynomial function of degree $n$ such that $f(x)$ yields a constant value for all $x$ in $\{1,2\dots n-1\}$.


Mostly, the reason I want to be able to find such a function is so when my math teacher says "find the function rule" and presents us with an obviously linear function I can give her some strange polynomial that just happens to give the correct answers for those values.

A polynomial degree $\leq n$ modeling $a_1,a_2, \cdots a_n$ would not be unique. Consider $f(1)=1$, $f(2)=4$. Obviously $f(x)=3(x-1)+1$ works, but we can let $f(x)=x^2$.
A polynomial of lowest degree that models $\{a_1,a_2,a_3,...a_n\}$ is unique. A polynomial of lowest degree that fits a set of points  is called the Lagrange Polynomial for that set of points. Here we want the Lagrange Polynomial for a very special set of points. A "closed form" is possible:
$$f(x)=\sum_{i=0}^{n-1} {x-1 \choose i} \Delta^i(1)$$
Here $\Delta^0(1)=f(1)$. Also $\Delta$ is defined as the operation mapping $f(x)$ to $f(x+1)-f(x)$. This operation is called the forward difference.Then $\Delta^i(1)$ is the operation iterated $i$ times then evaluated at $1$.
Ex: Find the Lagrange polynomial for $1,3,5$. 
$$\color{red}{1},3,5$$
Taking forward differences once gets.
$$\color{red}{2},2$$
Twice,
$$\color{red}{0},$$
This gives a Lagrange polynomial,
$$f(x)=\color{red}{1}{x-1 \choose 0}+\color{red}{2}{x-1 \choose 1}+\color{red}{0}{x-1 \choose 2}$$
$$=2x-1$$
Using this formula, you can troll your teacher. For example if your teacher asks find the rule for $1,3,5,$ then you can use the formula to find a rule for a sequence that goes $1,3,5,\pi$.
$$1,3,5,\pi$$
$$2,2,\pi-5$$
$$0,\pi-7$$
$$\pi-7$$
This gives,
$$f(x)=1+2{x-1 \choose 1}+0+(\pi-7){x-1 \choose  3}$$
$$=1+2(x-1)+(\pi-7)\frac{(x-1)(x-2)(x-3)}{3!}$$
