Alan Cobham. The intrinsic computational difficulty of functions. Logic, methodology and philosophy of science, Proceedings of the 1964 International Congress, edited by Yehoshua Bar-Hillel, Studies in logic and the foundations of mathematics, North-Holland Publishing Company, Amsterdam 1965, pp. 24–30.

Alan Cobham. The intrinsic computational difficulty of functions. Logic, methodology and philosophy of science, Proceedings of the 1964 International Congress, edited by Yehoshua Bar-Hillel, Studies in logic and the foundations of mathematics, North-Holland Publishing Company, Amsterdam1965, pp. 24–30. - Volume 34 Issue 4