It is shown that for each
0 < p < q < ∞ the space Lp(0,∞) + Lq(0,∞), defined as in Interpellation Theory, is
universal for the class of all Orlicz function spaces Lψ with Boyd indices strictly
between p and q (i.e. every Orlicz function space Lψ is order-isomorphically
embedded into Lp(0,∞) + Lq(0,∞)). The extreme case of spaces having
Boyd indices equal to p or q is also studied. In particular every space
Lr(0,∞) + Ls(0,∞) embeds isomorphically into the sum Lp(0,∞) + Lq(0,∞) for any
0 < p ≤ r ≤ s < q < ∞.
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