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PDF - Universiteit Twente

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Chapter 6. Norm relations using Laguerre polynomials<br />

In Chapter 4 we derived equation (6.12) in Lemma 4.13. In Theorem 4.14<br />

we used this equation to obtain a norm relation between semigroups and<br />

their cogenerators. To deal with the −x0 term on the right-hand side of<br />

equation (6.12), we compared the difference of two semigroups.<br />

If such a norm relation can be obtain using equation (6.13) is not a easy<br />

question, since there is no easy trick to deal with the two extra terms on<br />

the right-hand side. The same holds for α ≥ 3.<br />

On the other hand, with equation (6.11) it is possible to obtain a norm<br />

equality between the semigroup and the cogenerator. In Section 6.3 this<br />

norm equality is derived as a special case of Theorem 6.11.<br />

6.3 Norm equality<br />

In Section 6.2 we derived a relation between the semigroup and the cogenerator<br />

in Lemma 6.7. In the case α = 1 we used equation 6.9 to obtain a<br />

norm relation, see Theorem 4.14.<br />

In this section we derive a relation between the semigroup and the cogenerator<br />

as well. However, now we add a t α term to the integral on the left-hand<br />

side of equation (6.14). In this way we can obtain a norm relation for general<br />

α. Note in this section α ∈ R.<br />

As in Section 6.2, we use the Laguerre polynomials to transform the semigroup<br />

into the cogenerator.<br />

Lemma 6.9 Let n ≥ 0, let α > −1 and let A generate a semigroup with<br />

growth bound ω < 1. Further let L α n(t) be the Laguerre polynomials, see<br />

(6.1), and let {bn}n≥0 be given by (6.4). Then for every x0 ∈ X the following<br />

relation holds,<br />

80<br />

� ∞<br />

bne<br />

0<br />

−t/2 t α L α n(t)e At/2 x0dt = A n d (I − A) −(α+1) x0, x0 ∈ X. (6.14)

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