Even a casual reader of OEIS knows that some sequences are quite nice and elegant (e.g. A000108) while others are very erratic and rather difficult (e.g. A000002). One way to explain the difference is to say that the former sequences count natural combinatorial objects, while the latter do not. But what exactly IS a natural combinatorial sequenc...
Pak, Igor (University of California, Los Angeles)
University of Minnesota, Institute for Mathematics and its Applications.