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Eigen value and eigen function of the homogeneous integral equation

\(u(t) = \lambda\displaystyle\int_{0}^{1} e^{t+s}\, u(s)ds\)

is :

  • A.\(\lambda = \dfrac{2}{e^{2} - 1}, u(t) = e^{t^{2}}\)
  • B.\(\lambda = \dfrac{2}{e^{2} - 1}, u(t) = e^{t}\)
  • C.\(\lambda = \dfrac{e^{2} - 1}{2}, u(t) = e^{t}\)
  • D.\(\lambda = \dfrac{e^{2} - 1}{2}, u(t) = e^{t^{2}}\)

Answer: B

The eigenvalue and eigenfunction are \(\lambda = \dfrac{2}{e^{2} - 1}\), \(u(t) = e^{t}\).

The kernel \(e^{t+s} = e^{t}e^{s}\) is separable, so \(u(t) = \lambda e^{t}C\) with \(C = \int_{0}^{1} e^{s}u(s)\,ds\). Substituting \(u = \lambda Ce^{t}\):

\[ \begin{aligned} C &= \lambda C\int_{0}^{1} e^{2s}\,ds = \lambda C\,\frac{e^{2} - 1}{2} \\ \lambda &= \frac{2}{e^{2} - 1} \end{aligned} \]

for a non-zero C. The eigenfunction is a multiple of \(e^{t}\), not \(e^{t^{2}}\), and the reciprocal value \((e^{2} - 1)/2\) is the integral itself, not the eigenvalue.

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