“…Typically, recursion operators are considered as integro-differential operators, [37,12,56,10,9,11,47,1,38,16,48,46,55], although this interpretation is accompanied by a number of difficulties, e.g., discussed in [14,23]. Another definition is proposed in [40,14,19,20] (see also [44] and references therein) and developed in [27,48,32,28,29,31,30]. It treats recursion operators as the Bäcklund autotransformations of the tangent (or linearized) coverings of the pdes.…”