“…One can find early references on DKP equations in [6]. The matrix form of RWE is also convenient for the formulation of higher derivative field equations [7], [8], [9], [10], fields with multi-spin [11], [12], [13], [14], [15], [16], Einstein gravity equations [17], fields in curved space-time [18], [19], [20] and quantum chromodynamics [21]. There is a vast number of papers devoted to DKP equations, and, therefore, we mention only some part of them.…”