So I am searching for a simple function $f(x_1,x_2)$ such that $x_1 \in \mathbb{N}$ and $ x_2 \in \mathbb{R} $, however $f$ should have a physical meaning.
I was thinking with functions like area, volume, however all these functions takes inputs as positive real numbers. No fucntion taking one varaible as anatural number has come to my mind. Would some one help me if he/she has some ideas.
Thank you in advance.
The restriction to the natural numbers suggests a number of objects. For example: what is the surface area of $x_1$ cubes glued together in a line, where the side length of each cube is $x_2$?
As a side note, you will more often see variables like $n$ for natural numbers and $x$ for real numbers; furthermore, the indices $x_1$ and $x_2$ suggest that these variables are of the same kind. It would therefore look clearer to me to write $f(n,x)$ instead of $f(x_1,x_2)$.