@tomleslie if you don't mind, is there any chance to ask this one following up question?

What would the reason for the expression 'xxx_a' turning out as an error, while the next expression 'xxx_1_a' is completed successfully, in the attached worksheet?

Download LDOs_m1.mw

@acer Thank you for your answer, using 'eval' command!

I found a command 'assign' working for the purpose.

Q20220416_m1.mw 

@tomleslie Thank you for the detailed explanation!

@ecterrab I went through your note. It is clear. Thank you!

@dharr Sorry for the long delay in answer. 

@tomleslie : I don't know the validity of the way of expression, because I'm not a mathematian. However, such approach (i.e., treating differentiation as an algebraic operator) is common and widely practiced in electrical engineering. In fact, another expression ('p_()') in the worksheet attached to my original message was my second attempt to see if it would work in that way. 

@tomleslie Thank you for your answer. Your answer clarified the usage of the 'D', but the reason I came up with the 'D' is for the illustrative purpose, while I did not know the correct syntax you pointed in your message. 

@moderator: This question is a step further from the previous one. What I was trying to ask here is how to get the expression, presented by 'desired' in the worksheet attached to my question. 

@tomleslie Thank you for your answer. I did not know that it is the way (a TABLE with one indexed entry,). A reason behind the way of expression in my question is that it is from another application (i.e., 'Mathpix'). It creates an expression in MathML with the subscript style. 

In Kwon Park 

Thank you for all your answers. dharr's answer worked. Please have a look at the worksheet example below. 

The index (or subscription) convention, '1d', '2d', '1q', and so on, is a widely used convention in power system analysis (synchronous machine analysis, in particular). Thus, I wanted to see how to make the expression look 'correctly'. 

Download Q20220121_m1.mw

@wswain I found this: Calculating the Dew-Point and Wet-Bulb Temperature of Air(Working with Thermophysical Data: Dew-Point and Wet-Bulb Temperature of Air - Application Center (maplesoft.com)) from Maple website. Besides, perhaps I might go with the table for now. Thank you for your additional information.

@Carl Love/@wswain Thank you for your attention to my question. In fact, what Bill explained was what I was thinking, but I saw that the vapor pressure still exists below the freezing point. Please have a look at table B.1.5 in 'Borgnakke/Sonntag Fundamentals of Thermodynamics/7ed'.  

@acer / @Kitonum: thank you for your answers and sorry for my mistake. I should have been more careful in terms of writing the expression. 

@tomleslie Thank you for your answer. What I was looking for is an API that would translate the numerical value to a more meaningful string. However, I did not try to use the API, 'PhaseSI()' at that moment. Probably the API, 'PhaseSI()', would be sufficient. 

@tomleslie : Thank you for your checking and the answer. I tested with the latest Maple (2021.1) and saw the expected results.