There is usually no reason not to include something in a thesis. Your thesis is a place for you to give a coherent account of all your work on a topic. There are no page restrictions, and it should serve as a reference document for future readers.

Many theses include a lot of review material, beyond just what might be contained in a literature survey. A thesis has to contain new material, of course, but it does not all need to be novel. There can be a lot of merit in giving detailed explanations of known results, if the reader is not necessarily going to be familiar with them in detatil. A derivation that has not previously appeared in the literature (even if the final result is known) is clearly suitable for inclusion.

There is usually no reason not to include something in a thesis. Your thesis is a place for you to give a coherent account of all your work on a topic. There are (usually) no page restrictions, and it should serve as a reference document for future readers.

Many theses include a lot of review material, beyond just what might be contained in a literature survey. A thesis has to contain new material, of course, but it does not all need to be novel. There can be a lot of merit in giving detailed explanations of known results, if the reader is not necessarily going to be familiar with them in detail. A derivation that has not previously appeared in the literature (even if the final result is known) is clearly suitable for inclusion.