He took various ideas of Hilbert on integral equations and combined these into the concept of a Hilbert space around 1905. Hilbert had studied integral equations with symmetric kernel in 1904.
Siya kinuha ng iba't-ibang mga ideya ng Hilbert sa integral equation at pinagsama ang mga ito sa konsepto ng isang Hilbert space sa paligid ng 1905. Hilbert had-aral ng integral equation na may timbang kernel sa 1904.