briefly discuss the selection of the space Si of test input signals. As seen from (A.3), operators are compared by examining their response on signals chosen from Si. In selecting this space, one should therefore be careful to select only those signals that are of greatest interest or that excite the modes of H1 and H2 that are of main impor- tance. For example, if one is interested in comparing the response of H1 and H2 to narrow-banded signals centered around the frequency fc, then it is often be sufficient to select Si = {cos(2ft), sin(2nft)}. In this case, it is clearly useless to select test signals that have a large part of their frequency content outside the frequency band around fc. Norms based on such a restricted test signal set Si often simplify devising approximations for H1 or H2. Note, however, that these approximations are only valid when operating on input signals belonging to the restricted set Si, e.g. they only hold for one particular frequency band. Expressing LPTV operator norms in terms of the corre- sponding HTM elements Computing the operator norm (A.3) and the related quantities needed for comparing operators comes down to computing the signal norms lu and |||||||||||||. In what follows, we present expressions for these norms in terms of the equivalent baseband components of u(t) and the elements of the HTM corresponding to H[.]. Let us be given a continuous-time signals u(t) for which we define (A.10) i.e. the square of the signal norm equals the energy in u(t). Given the signal's equiva- lent baseband components Um(t), where u(1)= (A.11) we find that (A.12) Using Parseval's equality [Arf85], we then obtain UU(mo( do . (A.13) Note that when the Um(jw) are bandlimited within [-wo/2,wo/2], (A.13) reduces to (A.14) (A.15) with U(jw) = U_1(jw) Uo(jw) U1(jw) ...]The vector containing the (bandlimited) equivalent baseband components. For |H||||2 we find a similar expres- sions by replacing U(jw) with H(j))(U(jw) in the expression above. For bandlimited equivalent baseband components Um(jw), th