By W. Eckhaus

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**Example text**

Then, by extension a function Eim)@,5 E [O,co)can be defined. We can next study the regular expansion E y y ) @= E'"'T,E:"'@, In Chapter 3 we shall show that, for a class of functions, and with special provisions for the sequence d;), one has the relation Ep)E(m)E(m)@= E ( m ) E ( m ) @ . x 5 5 x This relation is the so-called asymptotic matching principle, and is fundamental to the method of matched asymptotic expansions. It would be very difficult, and cumbersome, to express such relations without the formalism of expansion operators.

We illustrate the procedure by the following elementary example: Consider 1 @=A x E [O,l]. X+E' We have and 0 - ELm)@ = o(E")for x E [ d , 11, Vd > 0. 1 to this example, one can take $ - &lim + 2 1 - CH. 2. Suppose that the local approximation EL)@ of @ is such that Tso@- E c ) @ = o(Sp)) for toE [O,A], V A > 0. ,n, there exist order functions The order functions Sp = o(1) such that Zp satisfy Proof and comments. 2 bis. Consider for that purpose, for each p = O,l,.. ,, n, @** = 1 ~ SpL [Tco@- E(snd@].

We find: =f(x) qm)@ + c ( - 1)P(:)"", m-l p=o - x E [d,l], Vd > 0, n 1 We analyse now the extended domains of validity as in Example 1, and obtain CD = Os(&(1-V)(m+1)), x E [EV,l], v v E (O,l), J y @ 0 - T,E:"d@= OS(&"("+ I)), x E [O,EP], v p E (OJ]. In order to achieve overlap one must be able to determine, for any integer k , integers m and n such that (1-v)(m+l) > k and p ( n + l ) > k while furthermore one must always satisfy the condition v > p. We find that the overlap is not strong for k > 0.