@vv  Thnaks!   expand is  more  stronger than simplify and value. It's weird.

@Carl Love 

I see, thank you！

@Pascal4QM Thanks it is useful!

@Carl Love 

When f (specific expression) is very complicated, in order to write concisely

@Carl Love  

I understand now, thank you so much！ Indeed,It can be generated by the transpositions (2, 3) and (3, 4)!

@dantopa   When we use the following code, we will get the right output. The example from vv.

I post  another problem of ChromaticNumber in the link below. 

 https://www.mapleprimes.com/questions/229567-Bug-Of-ChromaticNumber

@tomleslie Thanks and maybe you are right. I still not know  immanent cause.

@Carl Love  Extremely grateful

@Carl Love  Thank you very much!  Unfortunately，I am not still familiar for creating  Array inside loop.  I have a question :  How to save these graphs  based on  using  iterator codes rather than naive method of building lists inside loop.

.

@Carl Love  Thanks!  By testing in my computer, your codes cost 0.375 seconds .  My codes cost 0.672 seconds .

It's much better!   I read the help of  NonIsomorphicGraphs. I noticed one  option output=iterator . 

 So I use output=iterator in order to avoid costing large RAM when complete sequence of graphs  is formed at once.  Sometimes it  even causes Maple  to crash.

@vv  Maple 2020 can do that!

@ Christopher2222  when I first use mapleprimes I don't know why my question was deleted quickly 

@ecterrab   I following your advice that input libname, we get following two sentences:

"D:/Program Files/Maple 2020/lib", "C:/Users/asus/maple/toolbox/Physics Updates/lib"

And I restart the Maple, it is OK! 

Thank you for your help

 If we use Mathematica, we will get nothing.  Maybe Mma considers difinition of limit of function.

@dharr 

I just replied to  acer   with some details.  Of course, I don't know how to programmatically change the color of the string output, that seems like another problem