Which topics did Rudolf Carnap study in his early research after earning his doctorate?

Following his doctoral studies, which concluded in 1921 with a dissertation on the concept of space, Rudolf Carnap engaged in several years of private research. During this early period, his work focused on the following areas:

• Logic: He conducted research in formal logic, eventually publishing the textbook Abriss der Logistik (1929) on symbolic or mathematical logic.1
• Foundations of Physics: He explored the fundamental principles underlying physical science.1
• Philosophical Problems: He authored various essays addressing specific problems related to time, causality, and further investigations into the nature of space.1