I am currently writing my PhD thesis, and I have published an article together with my supervisor. The third chapter of my PhD thesis (1/3 of my whole thesis) is basically the paper. I plan to include the 30 pages of the article of the content that I have produced almost unchanged, except that it fits to my thesis layout, and I will remake the images, remove the introduction and some other minor changes. I also will add some new content, but this is mostly 3-4 pages.
My first question is, how do you actually cite 30 pages? Do you add a sentence at the beginning of the chapter like that?
The content of this chapter has been published before in ...
My second question is, should I ask the "author service" of the journal where my original article has been published if it is okay if I reuse most of the content from the article in my PhD thesis?
There is nothing regarding this issue in my PhD regulations.
[Edit]Some additional information: I am from Germany, I am doing my PhD in math, and I published the article in the journal Numerical Linear Algebra with Applications.