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Let \(f(x) = x^{2}\) and \(g(x) = x^{2} \log x\) on \(|x| \gt 0\). Then :

  • A.\(f, g\) are linearly dependent
  • B.\(f, g\) are linearly independent but cannot be solutions of an ordinary differential equation
  • C.\(f, g\) can be solutions of an ordinary differential equation
  • D.Wronskian of \(f\) and \(g\) is \(x^{2}\)

Answer: C

The correct statement is that \(f\) and \(g\) can be solutions of an ordinary differential equation.

They are linearly independent, since \(g/f = \log x\) is not constant, which rules out 'linearly dependent'. Their Wronskian is

\[ \begin{aligned} W &= x^{2}(2x\log x + x) - 2x \cdot x^{2}\log x \\ &= x^{3} \end{aligned} \]

so it is not \(x^{2}\). Since \(W \neq 0\) for \(x \neq 0\), they form a fundamental pair for a second-order linear equation. Indeed \(x^{2}\) and \(x^{2}\log x\) are the solutions of the Euler equation \(x^{2}y'' - 3xy' + 4y = 0\), whose indicial equation \(m^{2} - 4m + 4 = 0\) has the repeated root 2. So 'cannot be solutions of an ordinary differential equation' is false.

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