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Is there a function or anything else that gives the same result on a set?

I want to know if there is a function, for example, that gives the same result on the interval $[10,15]$. I want to have these function values

$f(10)=25,\, f(11)=25,\, f(12)=25,\, f(13)=25,\, f(14)=25,\, f(15)=25$

Thanks.

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    what are you hoping to achieve with this? what's the larger picture? are you familiar with the concept of [level sets](http://en.wikipedia.org/wiki/Level_set)?2012-04-30

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Actually, following Dejan Govc's lead, you could take any Fourier Series $ f(x) = a_0 + \sum_{n=1}^\infty a_n \cos\left(2\pi nx\right) + b_n \sin\left(2\pi nx\right) $ whose cosine coefficients $a_n$ sum to $25$ (the $b_n$ are completely free). The constant function corresponds to $(a_0,a_1,\dots)=(25,0,0,\dots)$ and Dejan's to $(0,25,0,\dots)$, both with all $b_n=0$. The complete function space would look like the set of periodic functions on the unit interval with the boundary condition $f(0)=f(1)=f(k)=25$ for all $k\in\mathbb{Z}$. This is a pretty large space of functions!