Question: word needed for definition

what do i call a homogenous  differential equation that is the linear sum of "N" differential of unique classification? ie, the implicit construction of a third homogenous differential by the summation of two known, is it the span of the solution sets of the first two or union? i prefer span because well that leaves the door open for multivariate differential basis definitions, non commutative groups like sets of square matrices and all of the other extra arousing subject content.

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