1. John R.Steel. Gödel’sprogram. Slides for a talk given at the CSLI meeting, Stanford University. 2013. https://math.berkeley.edu/∼steel/talks/stanford2013copy.pdf.
2. Harvey Friedman. Proof-theoretic Degrees. Foundations of Mathematics (FOM) mailing list. Post 20 in Friedman’s FOM series on topics in the foundations of mathematics. 1998. https://cs.nyu.edu/pipermail/fom/1998-August/001929.html.
3. Andrés E. Caicedo. (Non?)-linearity of the consistency strength ordering in ZF. MathOverflow answer. 2011. https://mathoverflow.net/q/59800.
4. Stephen G. Simpson. “The Gödel Hierarchy and Reverse Mathematics” (2009). Manuscript published on the author’s web page. http://www.personal.psu.edu/t20/papers/gh/.
5. John R. Steel. “Gödel’s program”. In: Interpreting Gödel. Cambridge Univ. Press, Cambridge, 2014, pp. 153–179.