1. Endrullis, J., Hendriks, D., Klop, J.: Degrees of streams. J. Integers 11B(A6), 1–40 (2011). Proceedings of the Leiden Numeration Conference 2010
2. Shallit, J.: Open problems in automata theory: an idiosyncratic view. LMS Keynote Talk in Discrete Mathematics, BCTCS (2014).
https://cs.uwaterloo.ca/shallit/Talks/bc4.pdf
3. Sakarovitch, J.: Elements Of Automata Theory. Cambridge University Press, Cambridge (2003)
4. Endrullis, J., Grabmayer, C., Hendriks, D., Zantema, H.: The Degree of Squares is an Atom (Extended Version). Technical report 1506.00884, arxiv.org, June 2015
5. Siefkes, D.: Undecidable extensions of monadic second order successor arithmetic. Math. Logic Q. 17(1), 385–394 (1971)