You say you "want x to always be a symbol", but all symbols will be replaced with their assigned values (OwnValues) if they exist, so this can't actually be done, per se. What you can do, however, is apply one of the various Hold* functions. Probably HoldForm is what you want in this case, and you don't need the Block for this:

TraditionalForm@
 TextCell[Column[{Row[{HoldForm[x], " = ", 5}]}, Spacings -> 0.5], 
  "Text", FontFamily -> "Palatino", FontSize -> 18]

This might have undesirable implications if you do further processing of the result, but I would need more information to help you address any of those issues.

Extra info about Block

Inside the Block, x will have its OwnValues temporarily overridden. But once the Block finishes executing the OwnValues for x will revert to its previous value. Execution continues until nothing in the expression changes, so any x remaining after Block finishes will get replaced by the "original" OwnValues definition.

Let's play with this:

x = 7;
Block[{x = 5},
 {OwnValues[x], Hold[OwnValues[x]]}]

This returns

{{HoldPattern[x] :> 5}, Hold[OwnValues[x]]}

Now try:

ReleaseHold@%

This returns

{{7 :> 5}, {HoldPattern[x] :> 7}}

Eric: I am writing text to display on the screen with a fixed format. Files for students of Differential Equations with interactions using Manipulate. If in a Block its local variables have no value, I don't understand why they are not treated as symbols. I know the answer is by design of the Mathematica team. It doesn't match my intuition. I see that if x is global with no value or x does not exist in the global my proposed code works. That is, I get what I want with OwnValues[x] = {}. HoldForm[x] inside the code always works but my code would be horrible and laborious. I basically don't want problems with global variables. Ernesto

HoldForm[x] inside the code always works but my code would be horrible and laborious.

I don't understand why this would be the case. It seems a lot simpler than creating a whole Block expression. The situation is that you have a symbol that, instead of being evaluated with its OwnValues, you want to display in a nice way. That's exactly what the *Form functions are intended to do.

I'm realizing that we're rat-holing on the wrong things. You titled your post, Trouble understanding the local variables of the Block command, and so I've been trying to explain how Block works. But that's not your actual problem. Your actual problem seems to be something like, How can I display mathematical expressions nicely when they involve symbols that might have OwnValues? And furthermore, you want an approach that is repeatable hundreds of times. I'm confident that there are ways to do that that are simpler and more readable than the approach you've taken. But I don't want to start giving you suggestions only to have you tell me "that works fine with one variable, but my context is complicated". At this point, I suggest that you give us a representative example of a formatted output that you'd like to be able to generate. Include information about which symbols have OwnValues.

Excellent! Thanks!

So, just to make sure I don't go down the wrong trail, I have some clarifying questions:

• Did you consider using the SlideShow functionality?
• Is the Block expression in Eric.nb the way you usually edit content? Or is that expression itself actually computed from some other input?
• It's not clear to me whether your variables are assigned values in the global context. Could you just make all of your variables Protected? Or do you actually need some of your variables to have values in one context, but then you need to display an expression that uses those variables as raw symbols? I was assuming the latter, but I'm not seeing examples of such conflict (although I haven't gone through Test.nb exhaustively yet).
• Have you gone through the guides/tutorials in the documentation? For example:
  • http://reference.wolfram.com/language/workflowguide/CreatingDocumentsAndPresentations.html
  • http://reference.wolfram.com/language/workflowguide/CreatingAndOrganizingInterfaces.html
  • http://reference.wolfram.com/language/tutorial/CustomInterfaceConstructionOverview.html

Ernesto, I'm very confused. I've looked through Test.nb more thoroughly, and I don't see any reason why any of the textual content can't be just static content. I mean, you could programmatically assemble a notebook with the content, maybe piece together larger content from smaller snippets, but I can't see any reason why it can't just be something like static text cells with embedded cells for the mathematical expressions. I see that you have some controls for navigation, some TabViews and so forth, but none of that seems to depend on any of the symbols being used in the expository content.

I started trying to refactor your example (dDiv, dEx, etc), but when I started matching it up to the content in Test.nb, it just wasn't making any sense. What am I missing? I feel like I've just completely misunderstood your question here. It doesn't seem to have any bearing on the content in Test.nb.

In fact, you can simplify it further by doing this:

Block[{x, y}, 
 TextCell[
  Column[{ToString[x''[t]/Sqrt[x + y] == 5 y , 
     TraditionalForm]  (*I want x to always be a symbol*)}, 
   Spacings -> 0.5] , "Text", FontFamily -> "Palatino", 
  FontSize -> 18]]

You do not need to do any detailed string manipulation and you can put in any arbitrarily complex expression.

Regards

Neil

Gracias Eric