My question is: How to arrive at the answer $x_k = a^{-\frac{1}{p}}$?
I differentiated the expression Lagrangian that the author has defined w.r.t lambda and equated it to zero giving me summation over k from 1 to a ${}$|| xk|| equal to 1.which is giving me back the constraint.
I don't know how to proceed futher in obtaining the solution $x_k = a^{-\frac{1}{p}}$.I don't know LaTeX.