Well, if you wanted to be precise you should specify for which indices x_i=y_i should hold. But if it&#x27;s clear from the context then that might be left out.<p>You&#x27;re definitely right that mathematics doesn&#x27;t really have things like variables though. You&#x27;re only allowed to define a symbol once, but then it doesn&#x27;t necessarily need to have a specific value (e.g. you&#x27;re allowed to define &#x27;x&#x27; and &#x27;y&#x27; implicitly using the equation &#x27;x = 2yx&#x27;, or less trivially &#x27;x^2 + y^2 = 1&#x27;).