="Eugene Salamin" <gene_salamin@yahoo.com> But can't -1 be a legitimate element of an integer sequence?
Yes, but a "canonical" OEIS sequence consists only of non-negative integers. Thus -1 is a convenient conventional marker for an exceptional value. When used the meaning is always documented in the entry, typically by adding text of the form "...or -1 if..." to the description. When a sequence truly is best defined with signed integers they are given as an addendum to the main canonical sequence consisting of their absolute values, and the entry is tagged with a special keyword. Supporting a variety of values might seem congenial, but it would complicate lookup and open Pandora's Box. See for example A193616, which would include "1/2", "?", "infinity" and "coffee cup"!