After being shot, Helly had been captured by the Russians towards the end of 1915 and after a spell in hospital was by then in the same prison camp Radó. However unlike Radó, who had only just begun his university studies, Helly was already a research mathematician who had made remarkable progress in his work on functional analysis, proving the Hahn - Banach theorem in 1912.
Pagkatapos na ng baril, Helly had been nakuha sa pamamagitan ng Russians sa katapusan ng 1915 at pagkatapos ng isang spell sa ospital ay sa pamamagitan ng pagkatapos ay sa bilangguan ang parehong kampo Radó. Subalit hindi katulad Radó, na lamang ang lamang nagsimula ang kanyang pag-aaral sa unibersidad, Helly ay isang pananaliksik dalubbilang na ginawa pambihira progreso sa kanyang trabaho sa functional analysis, proving ang Hahn - Banach teorama sa 1912.