New posts in Second Order Logic

CH holds in V if and only if CH is actually true, for V a model of ZFC2

What axiom system for the complex numbers is categorical?

What is Tarski’s definition of real number multiplication?

How are sets defined in reverse mathematics?

Categoricity of second order theories - precisely what does it mean?

Well-ordering theorem and second-order logic

Which logic is stronger? SOL or $\frak{L}_{\infty,\infty}$?