Mathematical journals that accept long papers (up to 100 pages) I have a problem with finding a suitable mathematical journal (maybe not of the highest level) that accepts longer paper (say 100 pages). I know only two such journals: Memoirs of AMS and Dissert.Math.
But both journals now are in Q1 quartile, so are very selective, without any chances to be accepted there.
In fact, "Dissertationes Mathematicae" was created in 1951 (as "Rozprawy Matematyczne") with the purpose of publishing good Ph.D. Theses and indeed some time ago it was quite real to be accepted there. But since 2019 when they entered to the Q1 quartile this journal accepts only papers of exceptional quality (so far higher than an average Ph.D. Thesis).
So, what would be your suggestion concerning journals for longer papers (at the moment I would not like to consider splitting the long paper into smaller pieces). Thanks for the advise (I suspect that many mathematicians had such problems in mind, so the collective experience had to elaborate some helpful answer here).
 A: Lecture Notes in Mathematics (Springer) has a minimum of 100 pages.  But it may not be "peer reviewed" in the conventional sense.
A: Another option to publish can be SpringerBriefs in Mathematics. This is a book series for books with 50 to 125 pages. All books are peer-reviewed.
A: There is a newly-formed journal Memoirs of the European Mathematical Society that has a minimum limit of 80 pages.
It will issue the first volume this year, so I cannot evaluate their quality standards.
A: The MDPI journal Mathematics states that:

Our aim is to encourage scientists to publish their experimental and
theoretical results in as much detail as possible. There is no
restriction on the length of the papers. The full account of the
research must be provided so that the results can be reproduced.

It's a Q2 journal, which could well be what you are looking for.
MDPI is an agressive publisher, you might want to read this discussion.
A: Publishing a paper 50-100 pages long can indeed be a problem.
However many journals actually like long papers (they are likely to have
higher citation rate:-) though they do not advertise this. Specifically I recommend the following course of action: look at some long papers in your area, and notice where they are published.
For more than 100 pages, there are several excellent online-only journals, which usually have no size restriction.
A: I recall here my answer to a similar question: a journal which accepts long papers and has also an illustrious history is the Journal published by Castelnuovo Institute of Mathematics of the Sapienza University of Rome, i. e. the "Rendiconti di Matematica e delle sue Applicazioni". I recall the statements one can read on the homepage of the Journal:

The Journal "Rendiconti di Matematica e delle sue Applicazioni" is regularly issued since 1914.
The Journal traditionally publishes high-quality research articles in Pure and Applied Mathematics and adheres to the EMS Code of Practice.


Articles of any length are considered for publication. Submission of surveys, of articles of foundational nature, of doctoral dissertations etc. is also encouraged.
Every article submitted is subjected to a first screening by the Editorial board: if the manuscript meets the journal’s basic requirements, it will be sent to a referee for a single blind peer review process. Once a paper is accepted it goes immediately into production and published online shortly after the approval of galley proofs.

A: Applied Categorical Structures https://www.springer.com/journal/10485/ seems to match the topic of your paper and fairly regularly publishes 70+ pages papers. It is a Q2 journal according to Scimago.
