A3.18

Nonlinear Waves
Part II, 2004

(i) Let Φ+(t)\Phi^{+}(t) and Φ(t)\Phi^{-}(t) denote the boundary values of functions which are analytic inside and outside the unit disc centred on the origin, respectively. Let CC denote the boundary of this disc. Suppose that Φ+(t)\Phi^{+}(t) and Φ(t)\Phi^{-}(t) satisfy the jump condition

Φ+(t)=t2Φ(t)+t1+α(t1+tt3),tC,\Phi^{+}(t)=t^{-2} \Phi^{-}(t)+t^{-1}+\alpha\left(t^{-1}+t-t^{-3}\right), \quad t \in C,

where α\alpha is a constant.

Find the canonical solution of the associated homogeneous Riemann-Hilbert problem. Write down the orthogonality conditions.

(ii) Consider the linear singular integral equation

(t+t1)ψ(t)+tt1πiCψ(τ)τtdτ=2+2α(1+t2t2)\left(t+t^{-1}\right) \psi(t)+\frac{t-t^{-1}}{\pi i} \oint_{C} \frac{\psi(\tau)}{\tau-t} d \tau=2+2 \alpha\left(1+t^{2}-t^{-2}\right)

where \oint denotes the principal value integral.

Show that the associated Riemann-Hilbert problem has the jump condition defined in Part (i) above. Using this fact, find the value of the constant α\alpha that allows equation ()(*) to have a solution. For this particular value of α\alpha find the unique solution ψ(t)\psi(t).