Undecidability involves the halting problem for Turing machines and in 1991, when Robinson was aged 80, he published Minsky's small universal Turing machine which describes a universal Turing machine with 4 symbols and 7 states.
Undecidability ay nagsasangkot ng pagtigil-tigil ang problema para sa Turing machine at sa 1991, kapag Robinson ay may edad na 80, siya nai-publish Minsky ng maliit na universal Turing machine na naglalarawan ng isang universal Turing machine na may 4 na mga simbolo at 7 estado.