http://dswarmsikhttkg7jgsoyfiqpj3ighupfrvuz5ri3lu5q2dlqyrpgk7ad.onion/notes/frege-ancestral.html
Abbreviations The second-order predicate calculus with which we began has been extended with: a primitive \(\#\) operator, so that \(\# F\) can be formulated to signify the number of \(Fs\) a new axiom, namely, Hume’s Principle, to govern the new terms. Frege’s analysis of numbers: \begin{align*}
&C_0 = [\lambda x \, x\neq x] \\
&C_1 = [\lambda x \, x= \# C_0] \\
&C_2 = [\lambda x \, x= \# C_0 \lor x= \# C_1] \\
&C_3 = [\lambda x \, x= \# C_0 \lor x= \#...