Recamán's SequenceStart at zero. Each step, first try to jump back by 1, then 2, then 3… but if that would land below zero or on a number you have already visited, jump forward by the same amount instead. Drawn as semicircles that alternate above and below a line, the rule’s restless hopping becomes a lace of nested arcs.Bernardo Recamán Santos posed it in 1991. It is still unknown whether every whole number is eventually visited — 19 is the smallest one never seen after billions of terms. A one-line rule with an open problem inside.