Which set existence axioms are needed to prove the separable Hahn-Banach theorem? - Douglas K. Brown; Stephen G. Simpson (Ann. Pure Appl. Log., 1986)

The Baire Category Theorem in Weak Subsystems of Second-Order Arithmetic. - Douglas K. Brown; Stephen G. Simpson (J. Symb. Log., 1993)

Vitali's Theorem and WWKL. - Douglas K. Brown; Mariagnese Giusto; Stephen G. Simpson (Arch. Math. Log., 2002)