# What are equivalence classes that would not have (1+1 and 2 ) or (2 x 3 and 6) in same classes?

Are there any ordered equivalence classes that can be used to distinguish 1+1 and 2 , On one side there is an operation and on the other side just a single number.

This becomes more obvious when dealing with factoring and multiplication , e.g. 17.19 = 323 , LHS contains more information than RHS but how to put an order on them?

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An equivalence relation on a set $S$ partitions $S$ into disjoint subsets. What is $S$ in your case?

If $S$ is a set of standard numbers, like $\mathbb R$ or $\mathbb N$, then you can't put $1+1$ and $2$ into separate equivalence classes, because they are the same element (of $\mathbb R$ or $\mathbb N$ or whatever).

But if your set $S$ is the set of strings that represent well-formed arithmetic expressions, then you can define an equivalence relation according to (for example) the number of arithmetic operations in the string. Then "$2$" is equivalent to "$323$", and "$1+1$" is equivalent to "$17 \times 19$".

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I suppose you could define two strings to be equivalent if they represent the same number and have the same number of arithmetic operations. Then $5\times6$ would be equivalent to $3\times10$ but not to $2\times3\times5$, nor to $5\times7$. – Gerry Myerson Oct 27 '11 at 11:26
But what their computational complexity? 1+1 can supposedly be calculated in 1 step , where as 2 is calculated in 0 steps. so 1+1 and 2 end up being in different computational complexity classes. Maybe I should have not used the equivalance classes. – Arjang Oct 27 '11 at 11:36

You could look at the set $\mathbb N\times\mathbb N$ of ordered pairs of natural numbers with equality as equivalence relation. The single number $2$ should then be written as $(2,0)$. For this equivalence relation, $(1,1)$ and $(2,0)$ are not equivalent.

On the set $\mathbb N\times\mathbb N$, there is then a weaker equivalence relation defined by: $(a,b)\sim (c,d)$ if and only if $a+b=c+d$.

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