From the time that Seifert took up his duties in Heidelberg until the start of World War II in 1939, he continued his collaboration was Threlfall. they exchanged letters and, as in previous years, spent holidays together working on mathematics. they published their second joint book in 1938 which was the monograph Variational calculus in the large which was a text on Morse theory.
Mula sa oras na Seifert took up ang kanyang tungkulin sa Heidelberg hanggang sa pagsisimula ng World War II sa 1939, siya ang pagpapatuloy ng kanyang pakikipagtulungan ay Threlfall. sila palitan titik at, tulad ng sa mga nakaraang taon, na ginugol ng bakasyon sama-samang gumawa sa matematika. sila ay nai-publish ang kanilang mga libro sa ikalawang pinaghugpungan 1938 na kung saan ay ang monograp Variational calculus sa ang malaki kung saan ay isang text sa Morse theory.