cp's OEIS Frontend

These rational numbers represent a FRACTRAN program capable of calculating any computable function.

If, when started at c*2^(2^k), the program stops at 2^(2^m), then c encodes the computable function f_c, and f_c(k) = m, where c, k and m are nonnegative integers.

In the linked work Conway lists some values of c (which he calls "catalog numbers") encoding various simple functions, including the (extremely large) value of c for computing the k-th digit in the decimal expansion of Pi.