# Is it possible to get any number from two numbers?

Is it possible to get any give number with a minimum value and a maximum value?

to output any number you desire?

for example

x ? y = z

x = any number between 0 and 9
? = any operator [+,-,/,*, square root, power of,etc...]
y = any number between 0 and 9
z = outcome is any number between 0 and 10,000 (ten thousand)


My second question is: is there a way or formula to do it in reverse for example,

578 = x ? y


So if I know what Z (or outcome is) can I calculate what the other factors can be to give me that outcome using a formula or algorithmic method?

Please ask away if you need further clarifications to my questions I will try my best to explain it further more.

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There are $10$ choices for $x$ and $10$ choices for $y$. Let's say you allow $10$ possible operators. Then you can only get at most $10\times10\times10 = 1000$ different results. So the answer is no. – Rahul Oct 25 '12 at 23:59
If I suppose increase the number of operators to lets say 252 or more, (I know you are thinking WTH) but an operator could also be replace with a function (I am thinking in computer terms here not so much in maths) in theory this is possible? right? – Val Oct 26 '12 at 0:11

This is a vague question. I'll restrict myself to power, roots and the standard binary operations.

If you restrict yourself to the integers (i.e. $x,\ y,\ z \in \mathbb{Z}$) then it's not too difficult to see that any prime larger than $18$ will not be representable. There's also the multiplicity argument given by Rahul Narain in the comments.

On the other hand, if we allow $x,\ y,\ z\in \mathbb{R}$ then any number between $1$ and $387420489 = 9^9$ will be representable by simply taking $x=9$ and varying $y$ from $0$ to $9$. The power function is continuous and any real number between the indicated limits will have a representation in the form of $x^y$. For numbers between $0$ and $1$ there are many representations. You can simply take for example $x=z$ and $y=0$ with addition.

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First question's answer: if your input is between $0,9$ then assuming operations of (+,-,* and ^)

let $v = 4.19180654858$ $$z \leq (9+9=18)\leq(9*9=81)\leq(9^v=10,000)$$

you can use symmetry to get the left hand side the following inequality:

$$-9^v \leq z \leq 9^v$$

So the answer of your first question is 'yes' but with the restriction shown above, that is many values of $x,y$ would result in values of $z>10,000$. For example $(x=1,y=9^7)$.

Second question's answer: Yes you can find $x,y$ however, you will get several value pairs of $(x,y)$ for a given $z$, so $x,y$ is not unique all the time. For example:

$$z=3$$

has the following pairs as a solution (this is a sample set): $(x=1, y=3)$, $(y=3,x=1)$, $(x=3,y=\frac{1}{3})$.

Even if you are looking for integer solutions only, you could get more than one.

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