Hartmanis–Stearns conjecture
In theoretical computer science and mathematics, the Hartmanis–Stearns conjecture is an open problem named after Juris Hartmanis and Richard E. Stearns, who posed it in a 1965 paper that founded the field of computational complexity theory (earning them the 1993 ACM Turing Award). An infinite word is said to be real-time computable when there exists a multitape Turing machine which (run without input) writes the successive letters of the word on its output tape, taking a bounded amount of time between two successive letters.
Source: Wikipedia — Hartmanis–Stearns conjecture (CC BY-SA 4.0)