[試題] 103下 林太家 偏微分方程式二 Test2

作者: t0444564 (艾利歐)   2015-06-05 14:40:51
課程名稱︰偏微分方程式二
課程性質︰數學研究所基礎課
課程教師︰林太家
開課學院:理學院
開課系所︰數學系、數學研究所、應用數學科學研究所
考試日期︰2015年04月28日(二),10:20-12:10
考試時限:110分鐘
試題 :
              Test 2                 4/28/2015
1. 20%            1,p
 Prove directly that if u∈W (0,1) for some 1 < p < ∞, then
          1-(1/p)  1  p  (1/p)
 |u(x)-u(y)|≦|x-y|    (∫|u'| dt)   for a.e. x,y∈[0,1].
               0
2. 20%
 Assume 1 < p < ∞, and U is bounded.
             1,p       1,p
 (i) Prove that if u∈W (U), then |u|∈W (U).
           1,p      + -  1,p
 (ii)Prove that u∈W (U) implies u, u ∈W (U), and
             +  / Du  a.e. on {u > 0}
            Du =
               \ 0   a.e. on {u≦ 0},
             - / 0  a.e. on {u≧ 0}
            Du =
               \ -Du  a.e. on {u < 0}.
       +
   (Hint: u = lim F_ε(u), for
        ε→0
                / (z^2+ε^2)^(1/2) - ε if z≧ 0
           F_ε(z):= 
                \ 0           if z < 0.)
             1,p
 (iii) Prove that if u∈W (U), then
          Du = 0 a.e. on the set {u = 0}.
 Integrate by parts to prove the interpolation inequality:
             2      2 (1/2)   2 2 (1/2)
          ∫|Du| dx ≦ C(∫u dx)  (∫|D u| dx)
           U       U      U
       ∞                          2   1
 for all u∈C (U). By approximation, prove this inequality if u∈H(U)∩H(U).
       c                             0
 3. 20%          0
  Fix α > 0 and let U = B(0,1). Show there exists a constant C, depending
  only on n and α, such that
                 2       2
               ∫u dx ≦ C ∫|Du| dx,
               U      U
  provided                  1
           |{x∈U|u(x)=0}|≧α, u∈H(U).
 4. 20%  1 n
  Let u∈C (R ). Prove that
      c
        1                    |Du(y)|
      

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