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Let \(V = C^{1}[0,1]\), \(X = (C[0,1], \|\ \|_{\infty})\) and \(Y = (C[0,1], \|\ \|_{2})\). Then \(V\) is

  • A.dense in \(X\) but NOT in \(Y\)
  • B.dense in \(Y\) but NOT in \(X\)
  • C.dense in both \(X\) and \(Y\)
  • D.neither dense in \(X\) nor dense in \(Y\)

Answer: C

\(V\) is dense in both \(X\) and \(Y\).

By the Weierstrass approximation theorem, polynomials are dense in \(C[0,1]\) under the supremum norm. Polynomials are continuously differentiable, so \(C^{1}[0,1]\), which contains them, is dense in \(X\).

For \(Y\), note that \(\|f\|_{2} = \left(\int_{0}^{1} |f|^{2}\right)^{1/2} \le \|f\|_{\infty}\). So any approximation that is close in the supremum norm is at least as close in the \(L^{2}\) norm, and \(V\) is dense in \(Y\) as well.

That rules out every option that denies density in either space.

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